unittests: do not use auto_init

[miri64]

Unittests should not use auto_init. In most use-cases it would be called multiple times in the test's setup-function anyways.

[Kijewski]

ACK, auto_init should already have been deactivated.

[Enoch247]

10 years after this PR, the question of why it was done came up. Perhaps in 10 more years, it will be asked again, and nobody will be around who remembers. So documenting that now. You are welcome those reading this in the year 2034. :)

The reason why we deactivated the auto_init in unittests has multiple reasons:

1. The unittests used to, and to a degree still are a „catch-all“ application, that includes a lot of modules in the hopes to catch conflicts between them (that's why they also can be build separately with make test-.
2. We might want to test module initialization in the unittests
3. auto_init might cause side-effects for certain tests that expect another module to be deactivated (I remember that there was a problem with the old xtimer module, but I do not remember the specfics)

The above was taken from here:
https://matrix.to/#/!pqHdpanAvkJvlCwUDE:matrix.org/$2et3pDijENpQgXui_RPIMWqjrFlkj7z7W-Ce4la41KQ?via=matrix.org&via=tu-dresden.de&via=utwente.io

[miri64]

@Enoch247 Thanks!