tests: fix failure after Git upgrade from 2.33.1 to 2.35.1

slapos/recipe/build/tests.py

@@ -566,8 +566,8 @@ repository = %s
     with self.assertRaises(CalledProcessError) as output:
       check_output([buildout], stderr=STDOUT)
     self.assertIn(
-        b"error: Could not fetch broken",
-        output.exception.output)
+        b"error: could not fetch broken",
+        output.exception.output.lower()) # old Git prints a C
 
     # this reset repo
     self.assertFalse(os.path.exists(os.path.join(sample_buildout, "parts", "git-clone")))