borg2: move test suite, rename test files?

[ThomasWaldmann]

Getting #6386 to work was a bit more complicated than expected and especially the need for a pytest.ini (or same options given on the cmdline) was a bit ugly.

So, guess we could rename borg/testsuite to tests (or so), so that pytest finds it by default.

Also check if the test files need renaming (like to test_something.py?), so they are also found by default.

[ThomasWaldmann]

Looks like primarily the test files need renaming:

https://docs.pytest.org/en/stable/explanation/goodpractices.html#conventions-for-python-test-discovery

So, it is either test_*.py or *_test.py to be found "by default".