Bug 1592813 - Increase number of test chunks for windows 7/debug wpt; r=egao

Avoid intermittent task timeouts by running in more test chunks.

Differential Revision: https://phabricator.services.mozilla.com/D51272

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

taskcluster/ci/test/web-platform.yml

@@ -46,7 +46,7 @@ web-platform-tests:
             linux.*/debug: 18
             macosx*/opt: 8
             macosx.*/debug: 16
-            windows10.*/debug: 18
+            windows.*/debug: 18
             windows10-aarch64/opt: 16
             default: 12
     max-run-time: