Bug 1242053 part 2 - [css-grid] Enable subgrid support for tests. r=dholbert

testing/profiles/prefs_general.js

@@ -153,6 +153,7 @@ user_pref("layout.css.report_errors", true);
 
 // Enable CSS Grid for testing
 user_pref("layout.css.grid.enabled", true);
+user_pref("layout.css.grid-template-subgrid-value.enabled", true);
 
 // Enable CSS 'contain' for testing
 user_pref("layout.css.contain.enabled", true);