Bug 1575135 - Fix mock_pkg_config failure case. r=nalexander

It only happens when things go badly, which is why it doesn't cause
problems, but when moving things around, triggering the error, we
currently get a formatting error rather than the actual error.

Differential Revision: https://phabricator.services.mozilla.com/D42602

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/test/configure/test_checks_configure.py

@@ -652,7 +652,7 @@ Java SDK directory or use '--with-java-bin-path={java-bin-dir}'
                 return 0, '-l%s' % args[1], ''
             if args[0] == '--version':
                 return 0, mock_pkg_config_version, ''
-            self.fail("Unexpected arguments to mock_pkg_config: %s" % args)
+            self.fail("Unexpected arguments to mock_pkg_config: %s" % (args,))
 
         def get_result(cmd, args=[], extra_paths=None):
             return self.get_result(textwrap.dedent('''\