[PP-2] Update JsComponentTest to subclass ComponentSourceTestBase for consistent test coverage

Overview

#3501290: Introduce unit test coverage for both ComponentSource plugins (Block + SDC) added unit tests for sdc and block ComponentSource plugins.

Missing: the js ComponentSource plugin.

Proposed resolution

• Change JsComponentTest to final class JsComponentTest extends ComponentSourceTestBase {.
• Generalize JsComponentTest::testRenderComponent() into ComponentSourceTestBase, hence modifying ComponentSourceTestBase::testRenderComponentLive() to cover it all

User interface changes

None.