Add centos6 tests back

.github/workflows/run-tests-pebble.yml

@@ -39,6 +39,14 @@ jobs:
         run: docker-compose up -d --build
       - name: Run test suite on Alpine using Bash 5
         run: test/run-test.sh bash5-0
+  test-centos76
+      runs-on: ubuntu-latest
+      steps:
+        - uses: actions/checkout@v1
+        - name: Build the docker-compose stack
+          run: docker-compose up -d --build
+        - name: Run test suite on CentOS6
+          run: test/run-test.sh centos6
   test-centos7:
       runs-on: ubuntu-latest
       steps: