speed up testsuite

[ThomasWaldmann]

Our testsuite runs rather slow, especially on github's macOS runners (likely because they are notoriously overloaded).

It would be nice to analyze why the testsuite is slow and how it could be made faster (preferable without losing functionality or making things more complicated).

[ThomasWaldmann]

one recent improvement was to speed up borg benchmark cpu testing, #9417.

[ThomasWaldmann]

archiver tests that create archives from src_dir files can also be slow:

• src_dir points to src/borg/archiver/
• a few dozen *.py files in there
• BUT: Python creates a __pycache__ subdir in there with *.pyc for all files times any Python version that was used with them.

I added a backup_files fixture with a few files that can be used for tests that do not need a big backup volume.