Update include-what-you-use expected return code

Newer IWYU versions return exit code 1 for errors instead of 3.
Updated expected return codes to match new behavior.

Author: pocc

tests/test_hooks.py

@@ -230,7 +230,7 @@ def generate_iwyu_tests(cls):
 """.format(
             cls.err_cpp
         ).encode()
-        iwyu_retcodes = [0, 0, 3, 3]
+        iwyu_retcodes = [0, 0, 1, 1]  # Newer iwyu returns 1 instead of 3 for errors
         iwyu_output = [b"", b"", iwyu_err_c, iwyu_err_cpp]
         scenarios = []
         for i in range(len(cls.files)):