Added gmp extension to test workflow

If was not already enabled by default, should enable faster testing
handling as it helps the phpseclib usage for OIDC tokens in test rocket
through.

.github/workflows/test-php.yml

@@ -16,7 +16,7 @@ jobs:
       uses: shivammathur/setup-php@v2
       with:
         php-version: ${{ matrix.php }}
-        extensions: gd, mbstring, json, curl, xml, mysql, ldap
+        extensions: gd, mbstring, json, curl, xml, mysql, ldap, gmp
 
     - name: Get Composer Cache Directory
       id: composer-cache