[Finder] Marked test skipped when ftp stream wrapper is not available

| Q               | A
| ---------------- | ---
| Bug fix?         | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

This should prevent test from failing on hhvm as ftp stream wrapper is not currently implemented.
See https://github.com/facebook/hhvm/issues/2108 for more details.