Bug 1306635 - Run linux64-ccov wpt in more test chunks; r=jmaher

Avoid intermittently exceeding the already very long max-run-time in some chunks.

Differential Revision: https://phabricator.services.mozilla.com/D39679

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

taskcluster/ci/test/web-platform.yml

@@ -46,6 +46,7 @@ web-platform-tests:
         by-test-platform:
             android.*: 18
             linux64-asan/opt: 18
+            linux64-ccov/debug: 24
             linux.*/debug: 18
             macosx*/opt: 8
             macosx.*/debug: 16