Expose details of test failures (e.g. actual value)

[caugner]

We have come across features in older Chrome versions where the collector reported the feature as unsupported, although the feature was supported:

1. Writing border-image-width value auto, but reading equivalent value auto auto auto auto
2. Writing backgorund-position value bottom, but reading equivalent value 50% 100%

In these cases, it would have helped if the collector would have shown details about the test expectation mismatch.

This applies to CSS property tests, but also to Web API tests that check if an exception is thrown.