Add an `.only` test configuration

[martijnthe]

It would be nice to be able to write...

Test(simple, xyz, .only = true) {
   ...

... which would run only this test case.

I would use it during development for the test I'm currently working on. I realize you can do this through CLI arguments as well, but that means I've have to change the test runner command all the time, which means a context switch away from the .c file I'm working on.

The idea was not my own, credit goes to: https://github.com/avajs/ava#running-specific-tests