test: fix test-child-process-send-returns-boolean

test-child-process-send-returns-boolean was unreliable in CI and
locally.

* use 'SIGKILL' for more reliable process termination
* replace callback with assert.ifError()
* increase interval in fixture from 500ms to 9999ms. It's only purpose
  is to keep the process from exiting.

Fixes: #20135

PR-URL: #20136
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Yuta Hiroto <hello@hiroppy.me>

test/fixtures/child-process-persistent.js

@@ -1 +1 @@
-setInterval(function() {}, 500);
+setInterval(function() {}, 9999);

test/parallel/test-child-process-send-returns-boolean.js

@@ -17,9 +17,9 @@ const subScript = fixtures.path('child-process-persistent.js');
   // Test `send` return value on `fork` that opens and IPC by default.
   const n = fork(subScript);
   // `subprocess.send` should always return `true` for the first send.
-  const rv = n.send({ h: 'w' }, (err) => { if (err) assert.fail(err); });
+  const rv = n.send({ h: 'w' }, assert.ifError);
   assert.strictEqual(rv, true);
-  n.kill();
+  n.kill('SIGKILL');
 }
 
 {