chore(jest-runner): Add info regarding timers to forcedExit message

When a test finishes while a Node.js timer is still active, the worker
process will have to be force-killed by jest because the active timer
will keep the event loop alive.

It is often not immediately clear that such timers are the issue for this
message and the suggested method of `--detectOpenHandles` will never
detect them. Add a helpful message to point the user to this potential
issue.

Ref: https://nodejs.org/api/timers.html#timeoutunref

CHANGELOG.md

@@ -17,6 +17,7 @@
 
 ### Chore & Maintenance
 
+- `[jest-runner]` Add info regarding timers to forcedExit message([#12083](https://github.com/facebook/jest/pull/12083))
 - `[*]` Replaced `substr` method with `substring` ([#12066](https://github.com/facebook/jest/pull/12066))
 
 ### Performance

packages/jest-runner/src/index.ts

@@ -273,7 +273,8 @@ export default class TestRunner {
           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 --detectOpenHandles to find leaks.',
+              'Try running with --detectOpenHandles to find leaks. ' +
+              'Active timers can also cause this, ensure that .unref() was called on them.',
           ),
         );
       }