Fix incorrect resource name generation (#9957)

Javadoc has this to say about ClassLoader#getResource(String): "The name of a resource is a '/'-separated path name that identifies the resource."
The test fails on Windows because its File.separator does not equal "/".

server/src/test/java/com/vaadin/server/AbstractClientConnectorTest.java

@@ -162,7 +162,7 @@ public class AbstractClientConnectorTest {
             if (!name.startsWith("com.vaadin.")) {
                 return super.loadClass(name);
             }
-            String path = name.replaceAll("\\.", File.separator)
+            String path = name.replace('.', '/')
                     .concat(".class");
             URL resource = Thread.currentThread().getContextClassLoader()
                     .getResource(path);