github-action: add prepare-source test to linting

This is inspired by PR #307, but uses github actions rather than
Travis.

Signed-off-by: Tom Gundersen <teg@jklm.no>

.github/workflows/tests.yml

@@ -25,6 +25,18 @@ jobs:
       - name: Check out code into the Go module directory
         uses: actions/checkout@v1
 
+      - name: Check that source has been prepared
+        run: |
+          ./tools/prepare-source.sh
+          if ! git diff --quiet --exit-code ; then
+            echo
+            echo "Please include these changes in your branch: "
+            git diff
+            exit "1"
+          else
+            exit "0"
+          fi
+
       - name: Install golangci-lint
         run: curl -sfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh| sh -s -- -b $(go env GOPATH)/bin v1.23.7