INT-3298: Updated tests to BDD style

[danoaky-tiny]

Ticket: INT-3298

Changes:

• Replaced agar style chain/step tests with usage of describe, context & it.
• Added TestHooks.ts which contains BDD hooks for creating generic component fixtures and TinyMCE editor specific fixtures. As well as a convenience function for running tests in version contexts: eachVersionContext.
• Added TestHelpers.ts for other misc/alien helper or utils.
• Added teardown: { destroyAfterEach: true } to the TestBed.InitTestEnvironment call, which simplifies fixture disposal.
• Removed TestStore.ts as it's no longer needed/used.
• Added an eslint override for *Test.ts & /test//*.ts files.
• Added more tests for reactive forms and on push change detection mode to FormControlTest.ts