Auto merge of #26437 - servo:reftest-report, r=jdm

Generalize the 2020 regression report to show local unexpected failures

Example usage:

```
./mach test-wpt --release --layout-2020 --log-raw /tmp/servo.log
./tests/wpt/reftests-report/gen.py /tmp/servo.log
firefox ./tests/wpt/reftests-report/report.html
```

Produces a report similar https://community-tc.services.mozilla.com/api/index/v1/task/project.servo.layout-2020-regressions-report/artifacts/public/regressions.html, but showing unexpected reftest failures. The CI-generated one shows Layout 2020 failures (expected or not) that succeed in Layout 2013.

etc/taskcluster/decision_task.py

@@ -260,12 +260,13 @@ def layout_2020_regressions_report():
         .with_dockerfile(dockerfile_path("base"))
         .with_repo_bundle()
         .with_script(
-            "python3 etc/layout-2020-regressions/gen.py %s %s"
+            "python3 tests/wpt/reftests-report/gen.py %s %s"
             % (CONFIG.tree_hash(), CONFIG.git_sha)
         )
         .with_index_and_artifacts_expire_in(log_artifacts_expire_in)
-        .with_artifacts("/repo/etc/layout-2020-regressions/regressions.html")
-        .find_or_create("layout-2020-regressions-report")
+        .with_artifacts("/repo/tests/wpt/reftests-report/report.html")
+        .with_index_at("layout-2020-regressions-report")
+        .create()
     )
 
 def macos_unit():

etc/taskcluster/decisionlib.py

@@ -157,6 +157,10 @@ class Task:
 
     with_extra = chaining(update_attr, "extra")
 
+    def with_index_at(self, index_path):
+        self.routes.append("index.%s.%s" % (CONFIG.index_prefix, index_path))
+        return self
+
     def with_treeherder_required(self):
         self.treeherder_required = True
         return self
@@ -291,7 +295,7 @@ class Task:
             if e.status_code != 404:  # pragma: no cover
                 raise
             if not CONFIG.index_read_only:
-                self.routes.append("index.%s.%s" % (CONFIG.index_prefix, index_path))
+                self.with_index_at(index_path)
             task_id = self.create()
 
         SHARED.found_or_created_indexed_tasks[index_path] = task_id