Use vaadin-snapshots for test projects so that context menu is found (#8237)

test/pom.xml

@@ -22,6 +22,16 @@
     </properties>
 
     <repositories>
+        <repository>
+            <id>vaadin-snapshots</id>
+            <url>https://oss.sonatype.org/content/repositories/vaadin-snapshots/</url>
+            <releases>
+                <enabled>false</enabled>
+            </releases>
+            <snapshots>
+                <enabled>true</enabled>
+            </snapshots>
+        </repository>
         <repository>
             <id>vaadin-addons</id>
             <url>http://maven.vaadin.com/vaadin-addons</url>