Run regenerateParser on CI as a minimal test

.gitlab-ci.yml

@@ -17,7 +17,11 @@ before_script:
 
 tests:
   stage: test
-  script: ./gradlew ${GRADLE_OPTIONS} check
+  script:
+    # We don't have proper tests for SableCC, so as a minimal test, run SableCC on itself twice.
+    # If this succeeds, we at least know that it's not completely broken.
+    - ./gradlew ${GRADLE_OPTIONS} regenerateParser
+    - ./gradlew ${GRADLE_OPTIONS} regenerateParser
 
 tests:jdk-11:
   extends: tests