hphp/test/slow/ext_datetime/date.php depends on specific tzdata version

### HHVM Version

HHVM 3.13.1 and newer.
### Standalone code, or other way to reproduce the problem

I tested the following code in two different versions of tzdata packages: 2015g-0ubuntu0.15.04 and 2015a-1:

``` php
date_default_timezone_set("America/Los_Angeles");
var_dump(date("T"));
var_dump(date("T", -5000000000));
```

This test produces different outputs depending on the package installed.
### Expected result

The expected result, according to the date.php's "expectation file" is:

```
string(3) "PDT"
string(3) "PST"
```

Note: this result is generated when using 2015a-1
### Actual result

```
string(3) "PDT"
string(3) "LMT"
```

Note: this result is generated when using 2015g-0ubuntu0.15.04 0
### Comments

The example presented in this issue was extracted from hphp/test/slow/ext_datetime/date.php. Currently, this test is failing on 2015g-0ubuntu0.15.04 (which is the newer version) but not in 2015a-1.
I think, there are some options to treat this issue:
- Update the test with the expected result compliant to the newest tzdata version.
- Set the CMAKE file to expect a specific tzdata version.
- Skip test if the tzdata package currently installed is different from what is expected (don't know if this is actually possible)