test: add test for fork() + shell

This commit verifies that the child_process fork() method does
not honor the shell option.

Refs: nodejs/node#15299
PR-URL: nodejs/node#15352
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

test/parallel/test-child-process-fork-no-shell.js

@@ -0,0 +1,20 @@
+'use strict';
+// This test verifies that the shell option is not supported by fork().
+const common = require('../common');
+const assert = require('assert');
+const cp = require('child_process');
+const expected = common.isWindows ? '%foo%' : '$foo';
+
+if (process.argv[2] === undefined) {
+  const child = cp.fork(__filename, [expected], {
+    shell: true,
+    env: { foo: 'bar' }
+  });
+
+  child.on('exit', common.mustCall((code, signal) => {
+    assert.strictEqual(code, 0);
+    assert.strictEqual(signal, null);
+  }));
+} else {
+  assert.strictEqual(process.argv[2], expected);
+}