[hphp/test/run] Sub path support in the directory aliases

Add support for mentioning sub-paths in the directory aliases `zend, slow, quick, zend_bad`

For example `zend/tests` translate to `hphp/test/zend/good/tests` directory. Similarly works for other aliases.

Sample run is below
```
bash$ hhvm hphp/test/run zend/tests -i /001 -v -r 
Running 4 tests in 3 threads
hphp/test/zend/good/tests/basic/001.php passed
hphp/test/zend/good/tests/func/001.php passed
hphp/test/zend/good/tests/lang/001.php passed
hphp/test/zend/good/tests/strings/001.php passed

All tests passed.

              |    |    |
             )_)  )_)  )_)
            )___))___))___)\
           )____)____)_____)\
         _____|____|____|____\\__
---------\      SHIP IT      /---------
  ^^^^^ ^^^^^^^^^^^^^^^^^^^^^
    ^^^^      ^^^^     ^^^    ^^
         ^^^^      ^^^

```

`.travis.yml` has zend tests further split in `Repo` mode to avoid timing out on slower Travis VMs