Add swift 5.6 linux images to tests as well.

Author: mattpolzin

.github/workflows/tests.yml

@@ -33,6 +33,11 @@ jobs:
           - swift:5.5-focal
           - swift:5.5-centos8
           - swift:5.5-amazonlinux2
+          - swift:5.6-xenial
+          - swift:5.6-bionic
+          - swift:5.6-focal
+          - swift:5.6-centos8
+          - swift:5.6-amazonlinux2
     container: ${{ matrix.image }}
     steps:
     - name: Checkout code