Allow skipping tests at run-time.

[vadz]

This is another request for a way to skip tests, but it's not quite the same as #355 because in my case the condition determining if the test can be run or should be skipped is dynamic and determined by the program itself (basically it connects to the database specified on the command line and does different things depending on the exact kind of RDBMS).

Currently I just test the condition and use WARN("Skipped because..."), but this is not ideal because the test still counts as passing when, in fact, it wasn't run at all.

[Xazax-hun]

The next level would be to have a declarative way of describing dependencies between tests. And when one is skipped or failed skip the dependent ones.

[ranty-fugue]

It would be really great to have this facility for a bunch of tests I have, as their viability is determined only at runtime.

[vadz]

Great, thanks for implementing this! Will it be only part of v3 or is there a chance of this being backported to v2?

[horenmar]

I am not backporting it, someone else could -> the code should not have many dependencies on other v3 code.