clean up space in testing framework

[ThomasWaldmann]

Just added some cleanup code into some test_create_* tests, see #8980.

Maybe this can be solved better within the testing framework, so that temporary borg repositories and caches, input data, extracted data are always automatically cleaned up after the test function is completed.

pytest also does this, but only with a (too) big delay (IIRC: after 5 pytests runs or so), so /tmp tends to run full before that.