[2021.1 compat] Compatibility fixes to testing classes

* Remove call to not needed `Extensions.registerAreaClass` API after it was removed in [this commit](JetBrains/intellij-community@cc31a97#diff-5b5191d65df57cc34c25265d1be88fc3daedb8529c3b9c462d29312fd8360941)
* Use `ExtensionsArea.registerExtensionPoint(extensionPointName, extensionPointBeanClass, kind)` API that is available for all supported IDE versions after `ExtensionsArea.registerExtensionPoint(extensionPointName, extensionPointBeanClass)` was removed in [this commit](JetBrains/intellij-community@cc31a97#diff-f6d29de2def2588a09dcabf39c8ff1022dd787537bc5f0ed8124e9ed97610112)

PiperOrigin-RevId: 374622744

testing/src/com/google/idea/testing/ServiceHelper.java

@@ -39,7 +39,7 @@ public static <T> void registerExtensionPoint(
       ExtensionPointName<T> name, Class<T> clazz, Disposable parentDisposable) {
     ExtensionsArea area = Extensions.getRootArea();
     String epName = name.getName();
-    area.registerExtensionPoint(epName, clazz.getName());
+    area.registerExtensionPoint(epName, clazz.getName(), ExtensionPoint.Kind.INTERFACE);
     Disposer.register(parentDisposable, () -> area.unregisterExtensionPoint(epName));
   }
 

testing/src/com/google/idea/testing/TestUtils.java

@@ -22,7 +22,6 @@
 import com.intellij.openapi.Disposable;
 import com.intellij.openapi.application.Application;
 import com.intellij.openapi.application.ApplicationManager;
-import com.intellij.openapi.extensions.Extensions;
 import com.intellij.openapi.fileTypes.FileTypeManager;
 import com.intellij.openapi.util.Disposer;
 import com.intellij.openapi.vfs.encoding.EncodingManager;
@@ -150,7 +149,6 @@ static void createMockApplication(Disposable parentDisposable) {
   }
 
   static MockProject mockProject(@Nullable PicoContainer container, Disposable parentDisposable) {
-    Extensions.registerAreaClass("IDEA_PROJECT", null);
     container = container != null ? container : new DefaultPicoContainer();
     return new MockProject(container, parentDisposable);
   }