Fix post/test/* modules' loadpath

Allows loading when pwd is not framework's install root

### Verification
 - [x] `msfconsole`
 - [x] `cd /tmp/`
 - [x] `loadpath /path/to/metasploit-framework/test/modules`
 - [x] `use post/test/file`
 - [x] **VERIFY** nothing exploded