Throw explicit exception if experimental_split_coverage_postprocessin…

…g is used without experimental_fetch_all_coverage_outputs

This closes bazelbuild#13185
According to bazelbuild@a445cda,
The flag --experimental_split_coverage_postprocessing depends on the flag
--experimental_fetch_all_coverage_outputs being enabled, but if the former one
is set without the latter one, Bazel throws a quite confusing NullPointerException.
Now we throw an explicit exception with proper error message.

src/main/java/com/google/devtools/build/lib/exec/StandaloneTestStrategy.java

@@ -111,13 +111,10 @@ public TestRunnerSpawn createTestRunnerSpawn(
       throws ExecException, InterruptedException {
     if (action.getExecutionSettings().getInputManifest() == null) {
       String errorMessage = "cannot run local tests with --nobuild_runfile_manifests";
-      throw new TestExecException(
-          errorMessage,
-          FailureDetail.newBuilder()
-              .setTestAction(
-                  TestAction.newBuilder().setCode(TestAction.Code.LOCAL_TEST_PREREQ_UNMET))
-              .setMessage(errorMessage)
-              .build());
+      throw createTestExecException(
+          TestAction.Code.LOCAL_TEST_PREREQ_UNMET,
+          "cannot run local tests with --nobuild_runfile_manifests"
+      );
     }
     Map<String, String> testEnvironment =
         createEnvironment(
@@ -761,6 +758,16 @@ public void finalizeCancelledTest(List<FailedAttemptResult> failedAttempts) thro
     }
   }
 
+  private static TestExecException createTestExecException(TestAction.Code errorCode, String errorMessage) {
+    return new TestExecException(
+        errorMessage,
+        FailureDetail.newBuilder()
+            .setTestAction(TestAction.newBuilder().setCode(errorCode))
+            .setMessage(errorMessage)
+            .build()
+    );
+  }
+
   private final class BazelTestAttemptContinuation extends TestAttemptContinuation {
     private final TestRunnerAction testAction;
     @Nullable private final TestMetadataHandler testMetadataHandler;
@@ -884,6 +891,13 @@ public TestAttemptContinuation execute()
             .setHasCoverage(testAction.isCoverageMode());
 
         if (testAction.isCoverageMode() && testAction.getSplitCoveragePostProcessing()) {
+          if (testAction.getCoverageDirectoryTreeArtifact() == null) {
+            // Otherwise we'll get a NPE https://github.com/bazelbuild/bazel/issues/13185
+            throw createTestExecException(
+                TestAction.Code.LOCAL_TEST_PREREQ_UNMET,
+                "coverageDirectoryTreeArtifact is null: --experimental_split_coverage_postprocessing depends on --experimental_fetch_all_coverage_outputs being enabled"
+            );
+          }
           actionExecutionContext
               .getMetadataHandler()
               .getMetadata(testAction.getCoverageDirectoryTreeArtifact());