test: make test-fs-watch-recursive more robust

The test was failing on OS X when run in parallel with:

```
tools/test.py --repeat=99 -J parallel/test-fs-watch-recursive
```

ENOENT was arising when trying to create a file in the directory created
with `fs.mkdtempSync()`. I do not know if this is a bug in OS X, a bug
in libuv, or something else. I also do not know this is partially or
completely the cause of the flakiness of the test in CI and locally
(when run as part of `make test`).

In any event, changing to `fs.mkdirSync()` resolved the issue.

test/parallel/test-fs-watch-recursive.js

@@ -16,7 +16,8 @@ const filenameOne = 'watch.txt';
 
 common.refreshTmpDir();
 
-const testsubdir = fs.mkdtempSync(testDir + path.sep);
+const testsubdir = path.join(testDir, 'watch_me');
+fs.mkdirSync(testsubdir);
 const relativePathOne = path.join(path.basename(testsubdir), filenameOne);
 const filepathOne = path.join(testsubdir, filenameOne);
 
@@ -34,7 +35,7 @@ watcher.on('change', function(event, filename) {
   watcherClosed = true;
 });
 
-if (process.platform === 'darwin') {
+if (common.isOSX) {
   setTimeout(function() {
     fs.writeFileSync(filepathOne, 'world');
   }, 100);