Make build-time test failures fatal again, if reproducible

Require at least 3 successes out of 5 for now.

debian/changelog

@@ -10,6 +10,8 @@ ostree (2017.10-1) UNRELEASED; urgency=medium
   * Add patches to make the tests pass again when /var/tmp is on tmpfs,
     which does not support user xattrs
   * Add patch to remove useless #! from bash completions
+  * Make build-time test failures fatal again, but only if they are
+    reproducible (at least 3 times out of 5) for now
 
  -- Simon McVittie <smcv@debian.org>  Tue, 29 Aug 2017 16:08:09 +0100