add test for new XHP bug

add test for new XHP bug

Summary:
This test demonstrates what's probably a new XHP bug. The test succeeds on the previous nightly HHVM build but fails on the latest:

```
FAILED: test/slow/xhp/static-var.php
001+ Fatal error: Class undefined: \:fb:thing in /home/jjergus/work/code/hhvm/hphp/test/slow/xhp/static-var.php on line 11
001- int(42)
002- int(45)
003- int(45)
004- int(48)
```

It must be caused by one of the recent commits: https://github.com/facebook/hhvm/commits/master?after=f88acfe5fc92d4c07b7c2148deec311b5044224c+0

I don't know enough about HackC/HHVM internals to determine which commit caused it, can you help me triage?

Reviewed By: kmeht

Differential Revision: D18455008

fbshipit-source-id: b343fa5f54b2fad1c4d2f24f11487044607c9802