test: fix flaky async-hooks/test-graph.signal

Make sure event loop remains open long enough for signal to be received.

PR-URL: #17509
Fixes: #14568
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

test/async-hooks/test-graph.signal.js

@@ -3,16 +3,16 @@
 const common = require('../common');
 if (common.isWindows) {
   common.skip('no signals on Windows');
-  return;
 }
 
 const initHooks = require('./init-hooks');
 const verifyGraph = require('./verify-graph');
-const exec = require('child_process').exec;
+const { exec } = require('child_process');
 
 const hooks = initHooks();
 
 hooks.enable();
+const interval = setInterval(() => {}, 9999); // keep event loop open
 process.on('SIGUSR2', common.mustCall(onsigusr2, 2));
 
 let count = 0;
@@ -33,7 +33,9 @@ function onsigusr2() {
   }
 }
 
-function onsigusr2Again() {}
+function onsigusr2Again() {
+  clearInterval(interval); // let the event loop close
+}
 
 process.on('exit', onexit);