feat(apollo-angular): Adds flushData method to TestingOperation

[fetis]

closes #1469
plus, some refactoring for tests

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• Try to include the Pull Request inside of CHANGELOG.md

I'm not sure about changelogs. Shall I do it manually?

[apollo-cla]

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[fetis]

any feedback on it?

[fetis]

@kamilkisiela could u plz take a look?