Add test for findExtensionAnnotation (preparation for #363)

pf4j/src/main/java/org/pf4j/AbstractExtensionFinder.java

         return new ExtensionWrapper<>(descriptor, pluginManager.getExtensionFactory());
     }
 
-    private Extension findExtensionAnnotation(Class<?> clazz) {
+    public static Extension findExtensionAnnotation(Class<?> clazz) {
         if (clazz.isAnnotationPresent(Extension.class)) {
             return clazz.getAnnotation(Extension.class);
         }

pf4j/src/test/java/org/pf4j/AbstractExtensionFinderTest.java

  */
 package org.pf4j;
 
+import com.google.common.collect.ImmutableList;
+import com.google.common.io.ByteStreams;
+import com.google.testing.compile.Compilation;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.pf4j.plugin.FailTestPlugin;
 import org.pf4j.plugin.TestExtensionPoint;
 
+import javax.tools.JavaFileObject;
+import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.HashSet;
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import static com.google.testing.compile.CompilationSubject.assertThat;
+import static com.google.testing.compile.Compiler.javac;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.mockito.Mockito.eq;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.when;
         assertEquals(1, result.size());
     }
 
+    @Test
+    public void findExtensionAnnotation() throws Exception {
+        Compilation compilation = javac().compile(ExtensionAnnotationProcessorTest.Greeting,
+            ExtensionAnnotationProcessorTest.WhazzupGreeting);
+        assertThat(compilation).succeededWithoutWarnings();
+        ImmutableList<JavaFileObject> generatedFiles = compilation.generatedFiles();
+        assertEquals(2, generatedFiles.size());
+
+        JavaFileObjectClassLoader classLoader = new JavaFileObjectClassLoader();
+        Map<String, Class<?>> loadedClasses = classLoader.loadClasses(new ArrayList<>(generatedFiles));
+        Class<?> clazz = loadedClasses.get("test.WhazzupGreeting");
+        Extension extension = AbstractExtensionFinder.findExtensionAnnotation(clazz);
+        assertNotNull(extension);
+    }
+
+   static class JavaFileObjectClassLoader extends ClassLoader {
+
+        public Map<String, Class<?>> loadClasses(List<JavaFileObject> classes) throws IOException {
+            // Sort generated ".class" by lastModified field
+            classes.sort((c1, c2) -> (int) (c1.getLastModified() - c2.getLastModified()));
+
+            // Load classes
+            Map<String, Class<?>> loadedClasses = new HashMap<>(classes.size());
+            for (JavaFileObject clazz : classes) {
+                String className = getClassName(clazz);
+                byte[] data = ByteStreams.toByteArray(clazz.openInputStream());
+                Class<?> loadedClass = defineClass(className, data,0, data.length);
+                loadedClasses.put(className, loadedClass);
+            }
+
+            return loadedClasses;
+        }
+
+        private static String getClassName(JavaFileObject object) {
+            String name = object.getName();
+            // Remove "/CLASS_OUT/" from head and ".class" from tail
+            name = name.substring(14, name.length() - 6);
+            name = name.replace('/', '.');
+
+            return name;
+        }
+
+    }
+
 }

pf4j/src/test/java/org/pf4j/ExtensionAnnotationProcessorTest.java

  */
 public class ExtensionAnnotationProcessorTest {
 
-    private static final JavaFileObject Greeting = JavaFileObjects.forSourceLines(
+    public static final JavaFileObject Greeting = JavaFileObjects.forSourceLines(
         "Greeting",
         "package test;",
         "import org.pf4j.ExtensionPoint;",
         "   String getGreeting();",
         "}");
 
-    private static final JavaFileObject WhazzupGreeting = JavaFileObjects.forSourceLines(
+    public static final JavaFileObject WhazzupGreeting = JavaFileObjects.forSourceLines(
         "WhazzupGreeting",
         "package test;",
         "import org.pf4j.Extension;",
         "    }",
         "}");
 
-    private static final JavaFileObject WhazzupGreeting_NoExtensionPoint = JavaFileObjects.forSourceLines(
+    public static final JavaFileObject WhazzupGreeting_NoExtensionPoint = JavaFileObjects.forSourceLines(
         "WhazzupGreeting",
         "package test;",
         "import org.pf4j.Extension;",
         "    }",
         "}");
 
-    private static final JavaFileObject SpinnakerExtension = JavaFileObjects.forSourceLines(
+    public static final JavaFileObject SpinnakerExtension = JavaFileObjects.forSourceLines(
         "SpinnakerExtension",
         "package test;",
         "",
         "public @interface SpinnakerExtension {",
         "}");
 
-    private static final JavaFileObject WhazzupGreeting_SpinnakerExtension = JavaFileObjects.forSourceLines(
+    public static final JavaFileObject WhazzupGreeting_SpinnakerExtension = JavaFileObjects.forSourceLines(
         "WhazzupGreeting",
         "package test;",
         "",