Bug 1609967 - [ci] Stand up a 'shadow-scheduler' task to run the 'relevant-test' optimizer, r=jmaher

Depends on D60527

Differential Revision: https://phabricator.services.mozilla.com/D60528

--HG--
extra : moz-landing-system : lando

taskcluster/ci/source-test/shadow-scheduler.yml

@@ -22,6 +22,17 @@ job-defaults:
         mach: taskgraph optimized -p task-id=$DECISION_TASK_ID --output-file /builds/worker/optimized_tasks.list
         sparse-profile: taskgraph
 
+relevant_tests:
+    description: Runs the relevant_tests optimization strategy instead of SETA.
+    treeherder:
+        symbol: SS(relevant_tests)
+    index:
+        product: source
+        job-name: shadow-scheduler-relevant_tests
+    worker:
+        env:
+            TASKGRAPH_OPTIMIZE_STRATEGIES: taskgraph.optimize:relevant_tests
+
 seta_15_180:
     description: Runs the seta_15_180 experimental optimization strategy
     treeherder:

taskcluster/taskgraph/optimize/__init__.py

@@ -312,8 +312,14 @@ register_strategy('test-inclusive', args=('skip-unless-schedules',))(Alias)
 register_strategy('test-try', args=('skip-unless-schedules',))(Alias)
 
 
-# Experimental strategy that replaces the default SETA with a version that runs
-# all tasks every 10th push or 2 hours.
+# Experimental strategies that run in 'shadow-scheduler' tasks.
+
+# Runs task containing tests in the same directories as modified files.
+relevant_tests = {
+    'test': Either('skip-unless-schedules', 'skip-unless-has-relevant-tests'),
+}
+
+# A variation of SETA that runs all tasks every 10th push or 2 hours.
 seta_15_180 = {
     'seta': Alias('seta_15_180'),
     'test': Either('skip-unless-schedules', 'seta_15_180'),