Provide a mechanism to skip tests at runtime

[tgross35]

This is a revival of #38 (cc @bwidawsk) with the following changes:

• Change from an Option<String> to a custom enum
• Print the message in output, as is done by libtest when #[ignore = "foo"] is used
• Drop the commits adding log reordering and "sloppy" comparison

Fixes https://github.com/LukasKalbertodt/libtest-mimic/issues/19

[bugadani]

This PR would be very useful in probe-rs. The project (among other things) runs tests on embedded devices, and libtest-mimic just orchestrates and provides the output. We allow aborting a test run with ctrl-c, but we do it by aborting the process, which causes some problems with some devices. This PR would allow us to more gracefully exit.

[tgross35]

@LukasKalbertodt would you be able to take a look at this?