Build/Test Tools: Use correct value for split_slow.

Follow up to [58597], [58602].

See #61213.

git-svn-id: https://develop.svn.wordpress.org/branches/5.8@58603 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

.github/workflows/phpunit-tests.yml

@@ -64,12 +64,12 @@ jobs:
             os: ubuntu-latest
             memcached: true
             multisite: false
-            split_slow: true
+            split_slow: false
           - php: '7.4'
             os: ubuntu-latest
             memcached: true
             multisite: true
-            split_slow: true
+            split_slow: false
     with:
       os: ${{ matrix.os }}
       php: ${{ matrix.php }}