test: fix flaky child-process-exec-kill-throws

Kill the child process with `SIGKILL` to make sure the child process
does not remain alive.

Fixes: #20139

PR-URL: #20213
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-exec-kill-throws.js

@@ -19,7 +19,8 @@ if (process.argv[2] === 'child') {
  };
 
  const cmd = `"${process.execPath}" "${__filename}" child`;
- const options = { maxBuffer: 0 };
+ const options = { maxBuffer: 0, killSignal: 'SIGKILL' };
+
  const child = cp.exec(cmd, options, common.mustCall((err, stdout, stderr) => {
  // Verify that if ChildProcess#kill() throws, the error is reported.
  assert.strictEqual(err.message, 'mock error', err);