`mocha -h` doesn't always display help

Pretty straightforward testcase:

```
zarel@Phenylalanine ~> mocha -h

  Usage: mocha [debug] [options] [files]

  [...]

zarel@Phenylalanine ~> mkdir test
zarel@Phenylalanine ~> echo "test" > test/mocha.opts
zarel@Phenylalanine ~> mocha -h
No test files found
```

In other words, if `test/mocha.opts` exists, `mocha -h` won't display help.