Fix iwyu filtering to only check 'add' section for implementation headers

Check only the 'should add these lines' section for implementation
detail headers, not the entire output which may include regular headers
in the 'full include-list' section.

Author: pocc

tests/test_hooks.py

@@ -558,12 +558,19 @@ def run_shell_cmd(cmd_name, files, args, _, target_output, target_retcode):
             actual = actual.replace(b"clang-diagnostic-return-mismatch", b"clang-diagnostic-return-type")
         # Filter iwyu suggestions for implementation detail headers on macOS
         if cmd_name == "include-what-you-use" and sys.platform == "darwin":
-            # Remove entire iwyu output if it only suggests implementation detail headers (starting with __)
+            # Check if output only suggests adding implementation detail headers (starting with __)
             lines = actual.split(b"\n")
-            # Check if all suggested headers are implementation details
-            suggested_headers = [l for l in lines if l.strip().startswith(b"#include <__")]
-            if suggested_headers and all(b"<__" in l for l in lines if b"#include <" in l and l.strip().startswith(b"#include")):
-                # All suggestions are for implementation headers - filter entire output
+            in_add_section = False
+            add_section_headers = []
+            for line in lines:
+                if b"should add these lines:" in line:
+                    in_add_section = True
+                elif b"should remove these lines:" in line or b"The full include-list" in line:
+                    in_add_section = False
+                elif in_add_section and line.strip().startswith(b"#include"):
+                    add_section_headers.append(line)
+            # 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""
         retcode = sp_child.returncode
         utils.assert_equal(target_output, actual)