test: fix assertion in test-watch-file.js

Because it is comparing two Date objects, an assertion in
test/pummel/test-watch-file.js would never fire even if the two objects
represented the same time. Use `assert.notDeepStrictEqual()` so that the
assertion fires if different Date objects represent the same time.

PR-URL: https://github.com/nodejs/node/pull/8129
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/pummel/test-watch-file.js

@@ -14,7 +14,7 @@ function watchFile() {
   fs.watchFile(f, function(curr, prev) {
     console.log(f + ' change');
     changes++;
-    assert.ok(curr.mtime != prev.mtime);
+    assert.notDeepStrictEqual(curr.mtime, prev.mtime);
     fs.unwatchFile(f);
     watchFile();
     fs.unwatchFile(f);