Generalize use of test_utils_interactive_sync

Dear RIOTers,

I would like to change the current tests to use the helper utility introduced in #11875 [1]. This tool basically allows synchronizing the start of the test application with the python test script by waiting in the application code for a `s` (start) character sent by the test script.

Currently to sync we mostly depend on resetting the board after the serial terminal is opened. This can cause an issue for some boards as listed in #12448 [2]. Namely boards where `make reset` doesn't work or where `make reset` may disrupt the serial connection (this will become more common with the use of `cdc_acm_stdio`).

Using the mentioned tool will allow syncing some of these troublesome cases (e.g.: failing tests go from ~76 to ~16 for z1), #12461 [3] applies those changes. This will change the behavior of most of the tests when ran manually since you would always have to send a start (`s`) character (unless using the shell and some special cases).

I've gotten some feedback on the PR, but I would like to know if any of you see a fundamental issue with this behavior change. Cheers,


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