* Slightly more robust SessionTest and cleanup
Improvement of #9311
I noticed that my `/tmp` directory got filled up with random `sess_*` files and empty `frss_test_sessions_*` directories after repeatedly running PHPUnit, which means there was no proper cleanup being done in the tests.
See the first commit for changes, since I also moved the test to a more appropriate directory name (alongside `tests/lib/Minz`, `tests/app/Minz` was newly added in #9311).
Before the PR:
* Check output of `ls -lah /tmp/{frss_test_sessions_*,sess_*}; rm -r /tmp/{frss_test_sessions_*,sess_*}` to confirm there are initially no temporary files existing.
* Run `composer run phpunit`.
* Check output of the previous command again to confirm there is a new `sess_*` file and an empty `frss_test_sessions_*` directory.
After the PR:
* Check output of `ls -lah /tmp/{frss_test_sessions_*,sess_*}; rm -r /tmp/{frss_test_sessions_*,sess_*}` to confirm that no temporary files exist initially.
* Run `composer run phpunit`.
* Check output of the previous command again to confirm there are no temporary files, since they are now being cleaned up correctly.
* Move SessionTest from tests/app/Minz to tests/lib/Minz
