Add check for go mod tidy in CI (#601)

*Issue #, if available:* N/A

*Description of changes:*

Adds module tidy check to CI, to prevent changes introducing issues. We
should be able to avoid PRs being needed in the future to rectify, like
this one: #599

By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice.

Signed-off-by: Daniel Carl Jones <djonesoa@amazon.com>

Author: dannycjones

.github/workflows/unit-tests.yaml

@@ -22,6 +22,9 @@ jobs:
     - name: Check style
       run: make check_style
 
+    - name: Check module tidy
+      run: make check_mod_tidy
+
     - name: Check licenses
       run: make check_licenses
 

Makefile

@@ -176,6 +176,10 @@ e2e: e2e-controller
 check_style:
 	test -z "$$(gofmt -d . | tee /dev/stderr)"
 
+.PHONY: check_tidy
+check_mod_tidy:
+	test -z "$$(go mod tidy --diff | tee /dev/stderr)"
+
 .PHONY: check_licenses
 check_licenses: download_go_deps
 	go tool github.com/google/go-licenses/v2 check --allowed_licenses ${ALLOWED_LICENSES} ./...