add prefer-lowest option to the tests workflow matrix

Author: patinthehat

.github/workflows/run-tests.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         os: [ubuntu-latest]
         php: [8.0, 7.4, 7.3]
-        stability: [prefer-stable]
+        stability: [prefer-stable, prefer-lowest]
 
     name: P${{ matrix.php }} - ${{ matrix.stability }} - ${{ matrix.os }}