patch 9.0.1679: Cleanup Tests from leftover files

patch 9.0.1679: Cleanup Tests from leftover files

Problem:    Tests may leave leftover files around
Solution:   Clean up tests and remove files

There were a few failures in 'linux (huge, gcc, testgui, true, true)'
e.g. here: https://github.com/vim/vim/actions/runs/5497376153/jobs/10018060156

,----
| Error detected while processing command line..script /home/runner/work/vim/vim/src/testdir/runtest.vim[585]..function RunTheTest[54]..Test_lvimgrep_crash[16]..TestTimeout[12]..VimLeavePre Autocommands for "*"..function EarlyExit[7]..FinishTesting:
| line   70:
| E445: Other window contains changes
| E937: Attempt to delete a buffer that is in use: Xtest_stable_xxd.txt
| E937: Attempt to delete a buffer that is in use: Xtest_stable_xxd.txt
| E937: Attempt to delete a buffer that is in use: Xtest_stable_xxd.txtmalloc(): unsorted double linked list corrupted
`----

Which is puzzling, because the Xtest_stable_xxd file should have been
long gone after test_crypt.vim is run (and definitely no longer be
staying around in test_quickfix.vim).

So try to clean up properly after a test script is run, just in case any
X<file> is still around. During testing, a found a few leftover files,
which I also fixed in the relevant test-file.

Unfortunately, the test workflow 'linux (huge, gcc, testgui, true,
true)' now seems to fail with 'E1230: Encryption: sodium_mlock()' in
test_crypt.vim. Hopefully this is only temporary.