Update test_examples.py to use subTest now that we have modern Pythons

simplify it to use f-strings while we're at it

tests/test_examples.py

@@ -6,8 +6,6 @@
 sys.path.insert(0, '.')
 from tests.test_util import run_exe, cpp_supported
 
-EMIT_ELAPSED_TIME = False
-
 # Runs all pycparser examples with no command-line arguments and makes sure they
 # run successfully (return code = 0), without actually verifying their output.
 class TestExamplesSucceed(unittest.TestCase):
@@ -16,19 +14,11 @@ def test_all_examples(self):
         root = './examples'
         for filename in os.listdir(root):
             if os.path.splitext(filename)[1] == '.py':
-                # TODO: It would be nice to use subTest here, but that's not
-                # available in Python 2.7
-                # Use it when we finally drop Python 2...
-                path = os.path.join(root, filename)
-                t1 = time.time()
-                rc, stdout, stderr = run_exe(path)
-                elapsed = time.time() - t1
-                if EMIT_ELAPSED_TIME:
-                    print('{}... elapsed: {}'.format(filename, elapsed))
-                self.assertEqual(
-                    rc, 0, 'example "{}" failed with stdout =\n{}\nstderr =\n{}'.format(filename, stdout, stderr))
-
+                with self.subTest(name=filename):
+                    path = os.path.join(root, filename)
+                    rc, stdout, stderr = run_exe(path)
+                    self.assertEqual(
+                        rc, 0, f'example "{filename}" failed with stdout =\n{stdout}\nstderr =\n{stderr}')
 
 if __name__ == '__main__':
-    EMIT_ELAPSED_TIME = True
     unittest.main()