Skip test which performs some unreliable floating point comparisons

[smcv: Modified to use g_test_skip() instead of omitting those test cases
completely, and allow them to be re-enabled with a Debian-specific
environment variable]

Co-authored-by: Simon McVittie <smcv@debian.org>
Bug: https://gitlab.gnome.org/GNOME/glib/issues/820
Forwarded: no

glib/tests/timer.c

@@ -33,6 +33,12 @@ test_timer_basic (void)
   volatile gdouble elapsed;
   gulong micros;
 
+  if (g_getenv ("DEB_ALLOW_FLAKY_TESTS") == NULL)
+    {
+      g_test_skip ("Not reliable due to floating-point rounding (glib#820)");
+      return;
+    }
+
   timer = g_timer_new ();
 
   g_timer_start (timer);