Bug 1147723 - Avoid non TEST_PASS/TEST_UNEXPECTED_FAIL output from test_files.py. r=gps

Instead, capture the output and check it's being printed.

python/mozbuild/mozpack/test/support/minify_js_verify.py

@@ -2,10 +2,16 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
+from __future__ import print_function
 import sys
 
 
 if len(sys.argv) != 4:
     raise Exception('Usage: minify_js_verify <exitcode> <orig> <minified>')
 
-sys.exit(int(sys.argv[1]))
+retcode = int(sys.argv[1])
+
+if retcode:
+    print('Error message', file=sys.stderr)
+
+sys.exit(retcode)

python/mozbuild/mozpack/test/test_files.py

@@ -4,7 +4,10 @@
 
 from mozbuild.util import ensureParentDir
 
-from mozpack.errors import ErrorMessage
+from mozpack.errors import (
+    ErrorMessage,
+    errors,
+)
 from mozpack.files import (
     AbsoluteSymlinkFile,
     DeflatedFile,
@@ -40,6 +43,7 @@ import sys
 import mozpack.path as mozpath
 from tempfile import mkdtemp
 from io import BytesIO
+from StringIO import StringIO
 from xpt import Typelib
 
 
@@ -795,10 +799,16 @@ class TestMinifiedJavaScript(TestWithTmpDir):
 
     def test_minified_verify_failure(self):
         orig_f = GeneratedFile('\n'.join(self.orig_lines))
+        errors.out = StringIO()
         min_f = MinifiedJavaScript(orig_f,
             verify_command=self._verify_command('1'))
 
         mini_lines = min_f.open().readlines()
+        output = errors.out.getvalue()
+        errors.out = sys.stderr
+        self.assertEqual(output,
+            'Warning: JS minification verification failed for <unknown>:\n'
+            'Warning: Error message\n')
         self.assertEqual(mini_lines, orig_f.open().readlines())