Use "test name" in filenames like screenshots

[ctron]

Currently there seems to exist no functionality to get the test name that is currently being processed. This however should be used to prefix things like filenames of screenshots.

One proposal I saw is to create a test wrapper (using a macro) which injects this into the test being run. This could be done using "thread local", to somehow inject this into the TestContext that we have anyway. So it would be available throughout the test.

I guess the fact that we are using async tests might make this a bit more complicated though.