Add a test that ./demo SimpleSample.java works correctly.

.github/workflows/build.yml

@@ -17,7 +17,7 @@ jobs:
       - name: Build and Test
         uses: gradle/gradle-build-action@v2
         with:
-          arguments: build conformanceTests
+          arguments: build conformanceTests demoTest
         env:
           SHALLOW: 1
       - name: Check out jspecify/samples-google-prototype

build.gradle

@@ -158,6 +158,22 @@ clean.doFirst {
     delete "${rootDir}/tests/build/"
 }
 
+tasks.register('demoTest', Exec)  {
+    group = 'verification'
+    shouldRunAfter conformanceTests
+    dependsOn assemble
+    inputs.files('demo', 'SimpleSample.java')
+    executable '/bin/sh'
+    args 'demo', 'SimpleSample.java'
+    ignoreExitValue = true
+    errorOutput = new ByteArrayOutputStream()
+    doLast {
+        if (!errorOutput.toString().startsWith("SimpleSample.java:7: error:")) {
+            throw new AssertionError("`./demo SimpleSample.java` did not run correctly. Error output:\n$errorOutput")
+        }
+    }
+}
+
 /*
  Spotless validates its formatters' dependencies eagerly, on project configuration.
  google-java-format depends on checker-qual, which is built by a subproject.