fix(artifacts/test): clear the ArtifactStore.instance in S3ArtifactSt… (

#1137)

* fix(artifacts/test): clear the ArtifactStore.instance in S3ArtifactStoreConfigurationTest

so other tests that assume ArtifactStore.getInstance is null (e.g. ExpectedArtifactTest) don't try to store an artifact and fail trying to communicate with s3 with an error like:

software.amazon.awssdk.core.exception.SdkClientException: Unable to marshall request to JSON: Parameter 'Bucket' must not be null
	at software.amazon.awssdk.core.exception.SdkClientException$BuilderImpl.build(SdkClientException.java:111)
	at software.amazon.awssdk.services.s3.transform.HeadObjectRequestMarshaller.marshall(HeadObjectRequestMarshaller.java:53)
	at software.amazon.awssdk.services.s3.transform.HeadObjectRequestMarshaller.marshall(HeadObjectRequestMarshaller.java:31)
	at software.amazon.awssdk.core.internal.handler.BaseClientHandler.lambda$finalizeSdkHttpFullRequest$0(BaseClientHandler.java:73)
	at software.amazon.awssdk.core.internal.util.MetricUtils.measureDuration(MetricUtils.java:50)
	at software.amazon.awssdk.core.internal.handler.BaseClientHandler.finalizeSdkHttpFullRequest(BaseClientHandler.java:72)
	at software.amazon.awssdk.core.internal.handler.BaseSyncClientHandler.doExecute(BaseSyncClientHandler.java:151)
	at software.amazon.awssdk.core.internal.handler.BaseSyncClientHandler.lambda$execute$1(BaseSyncClientHandler.java:82)
	at software.amazon.awssdk.core.internal.handler.BaseSyncClientHandler.measureApiCallSuccess(BaseSyncClientHandler.java:179)
	at software.amazon.awssdk.core.internal.handler.BaseSyncClientHandler.execute(BaseSyncClientHandler.java:76)
	at software.amazon.awssdk.core.client.handler.SdkSyncClientHandler.execute(SdkSyncClientHandler.java:45)
	at software.amazon.awssdk.awscore.client.handler.AwsSyncClientHandler.execute(AwsSyncClientHandler.java:56)
	at software.amazon.awssdk.services.s3.DefaultS3Client.headObject(DefaultS3Client.java:5438)
	at com.netflix.spinnaker.kork.artifacts.artifactstore.s3.S3ArtifactStoreStorer.objectExists(S3ArtifactStoreStorer.java:132)
	at com.netflix.spinnaker.kork.artifacts.artifactstore.s3.S3ArtifactStoreStorer.store(S3ArtifactStoreStorer.java:90)
	at com.netflix.spinnaker.kork.artifacts.artifactstore.ArtifactStore.store(ArtifactStore.java:39)
	at com.netflix.spinnaker.kork.artifacts.model.ExpectedArtifact.store(ExpectedArtifact.java:160)
	at com.netflix.spinnaker.kork.artifacts.model.ExpectedArtifact.<init>(ExpectedArtifact.java:56)
	at com.netflix.spinnaker.kork.artifacts.model.ExpectedArtifact$ExpectedArtifactBuilder.build(ExpectedArtifact.java:44)
	at com.netflix.spinnaker.kork.artifacts.model.ExpectedArtifactTest.testReferenceMatches(ExpectedArtifactTest.java:205)

Yes, it's possible to configure a bucket name in S3ArtifactStoreConfiguration, but the more fundamental issue is having tests leaving a clean slate when they're done.

* refactor(artifacts/test): use @AfterEach to protect against other tests polluting state

kork-artifacts/src/test/java/com/netflix/spinnaker/kork/artifacts/artifactstore/s3/S3ArtifactStoreConfigurationTest.java

@@ -23,11 +23,13 @@
 import com.netflix.spinnaker.kork.artifacts.artifactstore.ArtifactStoreURIBuilder;
 import com.netflix.spinnaker.kork.artifacts.artifactstore.NoopArtifactStoreGetter;
 import com.netflix.spinnaker.kork.artifacts.artifactstore.NoopArtifactStoreStorer;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.TestInfo;
 import org.springframework.boot.context.annotation.UserConfigurations;
 import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+import org.springframework.test.util.ReflectionTestUtils;
 import software.amazon.awssdk.services.s3.S3Client;
 
 class S3ArtifactStoreConfigurationTest {
@@ -41,6 +43,16 @@ void init(TestInfo testInfo) {
     System.out.println("--------------- Test " + testInfo.getDisplayName());
   }
 
+  @AfterEach
+  void cleanup() {
+    // Clear ArtifactStore.instance so we don't leave lingering state for other
+    // tests that assume it's null.
+    //
+    // Note that ArtifactStore.setInstance(null) doesn't set instance to null
+    // if it's already set.
+    ReflectionTestUtils.setField(ArtifactStore.class, "instance", null);
+  }
+
   @Test
   void testArtifactStoreS3Disabled() {
     runner