Run book cronjob unit tests are run in make test

This makes sure that the cronjob's tutorial's tests are run in make
test, which should avoid breaking the book again.

Makefile

@@ -88,7 +88,7 @@ golangci-lint:
 ##@ Tests
 
 .PHONY: test
-test: test-unit test-integration test-testdata ## Run the unit and integration tests (used in the CI)
+test: test-unit test-integration test-testdata test-book ## Run the unit and integration tests (used in the CI)
 
 .PHONY: test-unit
 test-unit: ## Run the unit tests
@@ -119,3 +119,7 @@ test-e2e-local: ## Run the end-to-end tests locally
 .PHONY: test-e2e-ci
 test-e2e-ci: ## Run the end-to-end tests (used in the CI)`
 	./test/e2e/ci.sh
+
+.PHONY: test-book
+test-book: ## Run the cronjob tutorial's unit tests to make sure we don't break it
+	cd ./docs/book/src/cronjob-tutorial/testdata/project && make test