Fixes a regression found by Lei Zhang.

Based on a patch by Thomas Kho.

But report a failed test if a mandatory function is missing from it.

But report a failed test if a mandatory function is missing from it.
Also warn if the hardware capabilities force us to skip some tests.

tests: Add skip(), a function to warn that tests were skipped due to limitations of the environment or missing dependencies.

Fix the code so it compiles without warnings.