tests: Improve error message if setting max processes to 1 fails

This has historically failed on Debian autobuilders, and as a result we
have had a patch for a long time that turned a failure here into a
g_test_skip(). It's not clear whether this still happens, so I'm now
assessing whether the patch can be dropped; but if the prlimit() call
can fail for whatever reason, it would be useful for the error message
to say what limit we were trying to set.

Signed-off-by: Simon McVittie <smcv@collabora.com>

glib/tests/thread.c

@@ -147,7 +147,8 @@ test_thread4 (void)
   nl.rlim_cur = 1;
 
   if (prlimit (getpid (), RLIMIT_NPROC, &nl, &ol) != 0)
-    g_error ("prlimit failed: %s", g_strerror (errno));
+    g_error ("setting RLIMIT_NPROC to {cur=%ld,max=%ld} failed: %s",
+             (long) nl.rlim_cur, (long) nl.rlim_max, g_strerror (errno));
 
   error = NULL;
   thread = g_thread_try_new ("a", thread1_func, NULL, &error);