IDEA-65747 Javadoc: Don't use attribute value on javadoc tag autocompletion

1. Tag name only is used during javadoc tag auto completion;
2. Corresponding test is added;
3. java-impl module config file is corrected in order to define test source root;

java/java-impl/java-impl.iml

@@ -4,6 +4,7 @@
     <exclude-output />
     <content url="file://$MODULE_DIR$">
       <sourceFolder url="file://$MODULE_DIR$/src" isTestSource="false" />
+      <sourceFolder url="file://$MODULE_DIR$/testSrc" isTestSource="true" />
     </content>
     <orderEntry type="module" module-name="boot" />
     <orderEntry type="inheritedJdk" />