test-probe flake: the fork fixture's pidfile can be read empty #113

What happens. a_process_an_lv2_plugin_left_in_its_own_session_is_gone_after_the_run (tests/probe_test.py) failed once in four runs of make test-probe with

pid = int(open(pidfile).read())
ValueError: invalid literal for int() with base 10: ''

Why. The fixture's forked process (tests/lv2_fixture.c, fault_once, fault fork) does setsid(), then fopen(pidfile, "w"), then fprintf. The runner kills the whole tree when the child is done, which is what the test asserts, and it can land between the fopen and the write: the file exists and is empty. The test only checks that the file exists before it reads the pid.

Fix. The fixture writes the pid to a temporary file and renames it onto STRESS_FORK_PIDFILE (tests/stress_plugin.c too, if it writes the same way). Then the file is either absent or holds the pid. The test can then tell "the plugin did not get as far as forking" apart from "the leftover was killed".

Seen on origin/main 5016567 plus the #109 branch. The test is unchanged on that branch.