warn if a worker had to be force exited

packages/jest-runner/src/index.ts

@@ -7,6 +7,7 @@
 
 import {Config} from '@jest/types';
 import {SerializableError} from '@jest/test-result';
+import chalk from 'chalk';
 import exit from 'exit';
 import throat from 'throat';
 import Worker from 'jest-worker';
@@ -185,7 +186,18 @@ class TestRunner {
       ),
     );
 
-    const cleanup = () => worker.end();
+    const cleanup = async () => {
+      const {forceExited} = await worker.end();
+      if (forceExited) {
+        console.log(
+          chalk.yellow(
+            'A worker process has failed to exit gracefully and has been force exited. ' +
+              'This is likely caused by tests leaking due to improper teardown. ' +
+              'Try running with --runInBand --detectOpenHandles to find leaks.',
+          ),
+        );
+      }
+    };
     return Promise.race([runAllTests, onInterrupt]).then(cleanup, cleanup);
   }
 }