[test] Add reftest helper method for browser tests. NFC

This makes it easy distinguish reference tests from other btests.

test/common.py

@@ -2010,7 +2010,7 @@ def run_browser(self, html_file, expected=None, message=None, timeout=None, extr
 
   # @manually_trigger If set, we do not assume we should run the reftest when main() is done.
   #                   Instead, call doReftest() in JS yourself at the right time.
-  def reftest(self, expected, manually_trigger=False):
+  def make_reftest(self, expected, manually_trigger=False):
     # make sure the pngs used here have no color correction, using e.g.
     #   pngcrush -rem gAMA -rem cHRM -rem iCCP -rem sRGB infile outfile
     basename = os.path.basename(expected)
@@ -2141,8 +2141,15 @@ def compile_btest(self, filename, args, reporting=Reporting.FULL):
       filename = test_file(filename)
     self.run_process([compiler_for(filename), filename] + self.get_emcc_args() + args)
 
+  def reftest(self, filename, reference, *args, **kwargs):
+    """Special case of `btest` that uses reference image
+    """
+    assert 'reference' not in kwargs
+    kwargs['reference'] = reference
+    return self.btest(filename, *args, **kwargs)
+
   def btest_exit(self, filename, assert_returncode=0, *args, **kwargs):
-    """Special case of btest that reports its result solely via exiting
+    """Special case of `btest` that reports its result solely via exiting
     with a given result code.
 
     In this case we set EXIT_RUNTIME and we don't need to provide the
@@ -2175,7 +2182,7 @@ def btest(self, filename, expected=None, reference=None,
     if reference:
       self.reference = reference
       expected = [str(i) for i in range(0, reference_slack + 1)]
-      self.reftest(test_file(reference), manually_trigger=manually_trigger_reftest)
+      self.make_reftest(test_file(reference), manually_trigger=manually_trigger_reftest)
       if not manual_reference:
         args += ['--pre-js', 'reftest.js', '-sGL_TESTING']
     else: