Split embedded tests into individual binaries

[andy31415]

As we add more and more tests, as single large test binary that can be flashed to devices is not practical anymore. We already are excluding some tests for "not fitting in flash" and even after that we make fixes like #33347 because insufficient flash on some parts.

More over, some tests need to override things - see #9630 where it describes how multiple definitions of ember functions end up in single binaries. Splitting tests would allow us to test using some overrides a bit better in the current setup (even though supporting injection properly would likely be better)

We should have each chip-test generate a separate flashable binary, the same like it is built on the host.

We likely should do this after the migration of all tests to pw_unit_test or make the pw_unit_test version build separate binaries.