ditch master nightly tests but add official 5.2 tests for focal, centos8, and amazonlinux and add nightlies for 5.3

Author: mattpolzin

.github/workflows/tests.yml

@@ -15,13 +15,16 @@ jobs:
         image:
           - swift:5.2-xenial
           - swift:5.2-bionic
-          - swiftlang/swift:nightly-master-focal
-          - swiftlang/swift:nightly-master-centos8
-          - swiftlang/swift:nightly-master-amazonlinux2
+          - swift:5.2-focal
+          - swift:5.2-centos8
+          - swift:5.2-amazonlinux2
+          - swiftlang/swift:nightly-5.3-xenial
+          - swiftlang/swift:nightly-5.3-bionic
+          - swiftlang/swift:nightly-5.3-focal
+          - swiftlang/swift:nightly-5.3-centos8
+          - swiftlang/swift:nightly-5.3-amazonlinux2
     container: ${{ matrix.image }}
     steps:
-    - name: Install dependencies if needed
-      run: ${{ matrix.depscmd }}
     - name: Checkout code
       uses: actions/checkout@v2
     - name: Run tests