Add Pharo 11 and 12 into CI

Author: dionisiydk

.github/workflows/tests.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        smalltalk: [ Pharo64-10 ]
+        smalltalk: [ Pharo64-12, Pharo64-11, Pharo64-10 ]
     name: ${{ matrix.smalltalk }}
     steps:
       - uses: actions/checkout@v2