tests: test g_cond_wait_until() under stress
tests: test g_cond_wait_until() under stress

This (dubious) testcase fails before the previous commit due to errno
being clobbered by the interrupted wait on the contended mutex.  The
previous commit fixes that.

The testcase is dubious because, in theory (as per POSIX),
g_cond_wait_until() is permitted to return TRUE at any time for any
reason, due to so-called "spurious wakeups".  Having a testcase that
asserts that the return value should be FALSE is therefore fundamentally
broken.  We do it anyway, though.

We're only really trying to test a bug in our homemade Linux/futex
implementation here, and it takes a fair amount of effort to actually
convince the old code to fail (including some system stuff which
probably isn't available on Windows).  There's also the spurious wakeup
situation mentioned above to worry about on other systems.  For all of
those reasons, this test is only enabled on Linux.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib