Bug 1550515 - [mochitest] Don't 'keep_open' a single mochitest when using --headless, r=gbrown

Differential Revision: https://phabricator.services.mozilla.com/D30536

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

testing/mochitest/mach_commands.py

@@ -140,8 +140,10 @@ class MochitestRunner(MozbuildObject):
             # refresh the page to pick up modifications. Therefore leave the browser
             # open if only running a single mochitest-plain test. This behaviour can
             # be overridden by passing in --keep-open=false.
-            flavor = getattr(options, 'flavor', 'plain')
-            if len(tests) == 1 and options.keep_open is None and flavor == 'plain':
+            if (len(tests) == 1
+                    and options.keep_open is None
+                    and not options.headless
+                    and getattr(options, 'flavor', 'plain') == 'plain'):
                 options.keep_open = True
 
         # We need this to enable colorization of output.