test: include all stdio strings for fork()

test-child-process-fork-stdio-string-variant was only testing 'pipe' for
its `stdio` value. Add `inherit` and `ignore`.

Also added a `common.mustCall()` to verify that the `message` event is
triggered.

test/parallel/test-child-process-fork-stdio-string-variant.js

@@ -12,16 +12,19 @@ const childScript = `${common.fixturesDir}/child-process-spawn-node`;
 const errorRegexp = /^TypeError: Incorrect value of stdio option:/;
 const malFormedOpts = {stdio: '33'};
 const payload = {hello: 'world'};
-const stringOpts = {stdio: 'pipe'};
 
 assert.throws(() => fork(childScript, malFormedOpts), errorRegexp);
 
-const child = fork(childScript, stringOpts);
+function test(stringVariant) {
+  const child = fork(childScript, {stdio: stringVariant});
 
-child.on('message', (message) => {
-  assert.deepStrictEqual(message, {foo: 'bar'});
-});
+  child.on('message', common.mustCall((message) => {
+    assert.deepStrictEqual(message, {foo: 'bar'});
+  }));
 
-child.send(payload);
+  child.send(payload);
 
-child.on('exit', common.mustCall((code) => assert.strictEqual(code, 0)));
+  child.on('exit', common.mustCall((code) => assert.strictEqual(code, 0)));
+}
+
+['pipe', 'inherit', 'ignore'].forEach(test);