Auto merge of #29746 - servo:always-run-layout-2020-in-main-workflow, r=atbrakhi

Run the Layout 2020 tests in main workflow

Also fix report_aggregated_expected_results.py which was reporting an error when there were no failing tests. This is more commonly an issue with Layout 2020 because if runs fewer tests and was causing builds to show up as failing even when they were not.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they change the build infrastructure.

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.github/workflows/main.yml

@@ -59,6 +59,7 @@ jobs:
     uses: ./.github/workflows/linux.yml
     with:
       layout: '2020'
+      wpt: 'test'
 
   build_result:
     name: homu build finished

etc/ci/report_aggregated_expected_results.py

@@ -105,8 +105,11 @@ class Item:
 def get_results(filenames: list[str], tag: str = "") -> Optional[Item]:
     unexpected = []
     for filename in filenames:
-        with open(filename, encoding="utf-8") as file:
-            unexpected += json.load(file)
+        try:
+            with open(filename, encoding="utf-8") as file:
+                unexpected += json.load(file)
+        except FileNotFoundError as exception:
+            print(exception)
     unexpected.sort(key=lambda result: result["path"])
 
     def is_flaky(result):