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 #6817) merges. Therefore it's better to extract this functionality to Core/src/Testing.

[bshaffer]

I'm not a huge fan of static calls. I think we would be better off putting this in Google\Cloud\Core\Testing\System\SystemTestCase, which is extended by Firestore and Datastore tests? This way we could override the function, and also not have to use static calls to TestHelpers. This has the additional benefit that we can also replace the static calls to Assert with standard TestCase functions:

$this->assertNan($actual);
$this->assertEqualsWithDelta($expected, $actual, 0.01);
$this->assertSame($expected, $actual);

[yash30201]

We heavily need them for unit testing as they are separated in files much more than system tests.

And since Unit tests extends the PHPUnit's TestCase, our only options are:

1. Go with static functions by implementing them in TestHelpers.
2. Create a new Trait in Core/src/Testing and use it in required unit tests and SystemTestCase