Normalize iwyu return code when filtering implementation header suggestions

When iwyu output is filtered because it only suggests implementation
headers, also normalize exit code 1 to 0 if expected output is empty.

Author: pocc

tests/test_hooks.py

@@ -572,6 +572,11 @@ def run_shell_cmd(cmd_name, files, args, _, target_output, target_retcode):
             # If all "add" suggestions are for implementation headers, filter entire output
             if add_section_headers and all(b"<__" in h for h in add_section_headers):
                 actual = b""
+                # Normalize return code since we filtered the suggestions
+                if sp_child.returncode == 1 and target_output.strip() == b"":
+                    sp_child = sp.CompletedProcess(
+                        sp_child.args, 0, sp_child.stdout, sp_child.stderr
+                    )
         retcode = sp_child.returncode
         utils.assert_equal(target_output, actual)
         assert target_retcode == retcode