Bug 2034604 - Add web-platform-tests-eme to macosx1500-aarch64-tests r=aryx

WPT Batch 4 — same pattern as D301506 (crashtest + wdspec),
D302779 (canvas + webcodecs), and the non-WPT cutovers.

Validation try push https://treeherder.mozilla.org/jobs?repo=try&revision=9cc569e29c79f588b1eda5e42266ec160ae9bcd4
returned 12/12 green, with both opt + debug eme tasks completing
cleanly on macosx1500-aarch64.

A comparison of WPT eme results between Intel macosx1470-64 and
M4 macosx1500-aarch64 confirms equivalent effective coverage:

  Intel macosx1470-64 opt-eme: 43 clearkey OK, 50 drm SKIP
  M4 macosx1500-aarch64 opt-eme: 48 clearkey OK, 47 drm SKIP + 4 TIMEOUT

Both platforms run clearkey tests (W3C standard, no real CDM
required) and skip the drm-* tests (Widevine-dependent). The 4
M4-specific TIMEOUTs match documented WPT metadata expectations
for macOS aarch64. CI has not had functional Widevine on macOS in
the recent past; this migration preserves the existing
(clearkey-only) coverage profile while moving load off Intel.

After this lands, a paired follow-up patch will remove
web-platform-tests-eme from macosx1470-64-tests to complete the
WPT Batch 4 cutover.

Note: if product wants functional Widevine CDM coverage on macOS
CI, that requires a separate CDM provisioning workstream and is
not addressed by or blocked by this migration.

Differential Revision: https://phabricator.services.mozilla.com/D303892

taskcluster/test_configs/test-sets.yml

@@ -488,6 +488,7 @@ macosx1500-aarch64-tests:
     - web-platform-tests
     - web-platform-tests-canvas
     - web-platform-tests-crashtest
+    - web-platform-tests-eme
     - web-platform-tests-wdspec
     - web-platform-tests-webcodecs
     - xpcshell