Add CI tests for defmt-test

[jonas-schievink]

Who tests the test-runner?

[Sh3Rm4n]

If I understand it correctly, this about two possible three test steps:

1. Check that the generated code is buildable
2. Introduce proc-macro unit tests which check generated output against expected output
3. Check if the code can be run (which is difficult for thumb* targets with the CI)

If I have looked up correctly, no CI job is run for defmt-test at the moment, so 1. could be implemented by introducing a CI job.

[japaric]

now that defmt-test is in the defmt repository it should be possible to run firmware that uses defmt-test in the emulator using qemu-run (or at least I hope that works).

We should check two scenarios: all defmt-test tests pass and one defmt-test test fails; we have firmware tests that panic here. We should also check that the output of defmt-test matches our expectations -- we use snapshot testing for that.

[japaric]

it should be possible to run firmware that uses defmt-test in the emulator using qemu-run (or at least I hope that works).

those firmware tests / examples live in the firmware/qemu folder.

Adding a new program that uses defmt-test to firmware/qemu/src/bin should cover your point (1).
Listing that program in this script should cover your point (3)

In general, we don't test proc-macro expansion against expected Rust code (your point (2)) in this repo