tests: hierarchical structuring of the test directory

[OlegHahm]

Contribution description

The list of directories inside the tests folder has become really, really long and difficult to oversee. Hence, I propose to introduce some sub-directories to logically order the tests into groups represented by directories.

[cladmi]

I fixed the other flake8 complains.

[cladmi]

A first note, is that the new tests directories should be added to https://github.com/RIOT-OS/RIOT/blob/bbc16cced2c994ec57e0e7808e788932407c8885/makefiles/app_dirs.inc.mk to be detected by murdock and compile_test.
To make this more flexible, I am thinking about adding sub-files to declare this applications patterns. Like tests could have a file saying include the files in this directories, and each directory says, there are applications in these ones.

I like the idea of re-organizing the tests but with the current state I have a bit mixed feeling as there are still a lot of tests directories and directories with tests inside so an in between situation. So maybe go even further with the directory re-organization.

For packages I am also thinking of putting the tests in each package directory to move toward self contained packages. But it would require the modular definition of APPLICATION_DIRS I think.

[MrKevinWeiss]

I like nesting. I think if we can get consensus from enough people then schedule a time to get this done because I can imagine hitting a moving target is difficult.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. If you want me to ignore this issue, please mark it with the "State: don't stale" label. Thank you for your contributions.