GitHub Action, CI testing: Check on Windows and Mac OS too

Author: pH-7

.github/workflows/test.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ 'ubuntu-latest' ]
+        os: [ ubuntu-latest, windows-latest, macOS-latest ]
         php-versions: [ '7.1', '7.2', '7.3', '7.4' ]
         phpunit-version: [ '9.5' ]
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.os }}