chore: Extract compareResult test helper method to Core

[yash30201]

With the refactoring of tests cleanly use dataproviders with multiple cases in the single testing method, we expect the test method to receive multitude of data types requiring same assertEqualts. Few edge cases arising in those situations are:

• Asserting NANs
• Asserting null is equal to ''
• Asserting floats with some delta

This requirement is both is Firestore and Datastore (actually "will be" until https://github.com/googleapis/google-cloud-php/pull/6817) merges. Therefore it's better to extract this functionality to Core/src/Testing.