test: cover thrown errors from exec() kill

This commit adds code coverage for the scenario where exec()
kills a child process, but the call to ChildProcess#kill()
throws an exception.

PR-URL: https://github.com/nodejs/node/pull/11038
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

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

@@ -0,0 +1,29 @@
+'use strict';
+// Flags: --expose_internals
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const internalCp = require('internal/child_process');
+
+if (process.argv[2] === 'child') {
+  // Keep the process alive and printing to stdout.
+  setInterval(() => { console.log('foo'); }, 1);
+} else {
+  // Monkey patch ChildProcess#kill() to kill the process and then throw.
+  const kill = internalCp.ChildProcess.prototype.kill;
+
+  internalCp.ChildProcess.prototype.kill = function() {
+    kill.apply(this, arguments);
+    throw new Error('mock error');
+  };
+
+  const cmd = `${process.execPath} ${__filename} child`;
+  const options = { maxBuffer: 0 };
+  const child = cp.exec(cmd, options, common.mustCall((err, stdout, stderr) => {
+    // Verify that if ChildProcess#kill() throws, the error is reported.
+    assert(/^Error: mock error$/.test(err));
+    assert.strictEqual(stdout, '');
+    assert.strictEqual(stderr, '');
+    assert.strictEqual(child.killed, true);
+  }));
+}