tests: Skip slow mainloop test on valgrind

The `mainloop` test suite takes about 775s on my machine under valgrind
with this test enabled, vs 50s without this test enabled.

This causes CI failures like
https://gitlab.gnome.org/GNOME/glib/-/jobs/5321882.

I’m not sure that valgrind will actually successfully reproduce the race
condition because it runs too slowly (but I haven’t verified that by
reverting the fix for the race).

In any case, you can still choose to run the test under valgrind with
`-m thorough`.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

glib/tests/mainloop.c

@@ -25,6 +25,7 @@
 #include <glib.h>
 #include <glib/gstdio.h>
 #include "glib-private.h"
+#include "gvalgrind.h"
 #include <stdio.h>
 #include <string.h>
 
@@ -2644,6 +2645,16 @@ test_simultaneous_source_context_destruction (void)
   SimultaneousDestructionTest **test;
   guint64 i;
 
+  /* The race in this test is very hard to reproduce under valgrind, so skip it.
+   * Otherwise the test can run for tens of minutes. */
+#if defined (ENABLE_VALGRIND)
+  if (RUNNING_ON_VALGRIND && !g_test_thorough ())
+    {
+      g_test_skip ("Skipping hard-to-reproduce race under valgrind");
+      return;
+    }
+#endif
+
   if (g_test_thorough ())
     {
       n_concurrent = 512;