bpo-43316: gzip: Fix sys.exit() usage. (GH-24652)

diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index 1bb8f7a..1dcfa2b 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -775,7 +775,7 @@ def test_decompress_infile_outfile(self):
 
     def test_decompress_infile_outfile_error(self):
         rc, out, err = assert_python_failure('-m', 'gzip', '-d', 'thisisatest.out')
-        self.assertIn(b"filename doesn't end in .gz:", err)
+        self.assertEqual(b"filename doesn't end in .gz: 'thisisatest.out'", err.strip())
         self.assertEqual(rc, 1)
         self.assertEqual(out, b'')