PHPUnit test suite fails on Windows

PHPUnit's own test suite has various failures on Windows currently.  There are two dominant causes:

1. `DIRECTORY_SEPARATOR` mismatches in PHPT tests.
1. Expected stack traces in PHPT tests do not account for an additional stack frame that is caused by `PHPUnit_Util_PHP_Windows::process()`.

Lastly, there's a difference in libxml behavior with regard to `PHPUnit_Util_XML::load()` on Windows, which causes configuration.xml tests to fail, because relative file paths are not resolved correctly - they're resolved base on the current working directory, instead of the directory in which the configuration file is contained; setting `DOMDocument::$documentURI` does not help in any way (but is required and MUST be set before loading the XML for XInclude to work).

This might be the only debatable change.

Technically, it not only hides the process isolation script from stack traces, but any PHP code that happens to be executed through a temporary file in the temporary directory.

However, even on the assumption that such an edge case exists in the wild in the first place, then additional stack frames like the following are not exactly helpful anyway:
```
C:\Users\sun\AppData\Local\Temp\PHP4B92.tmp:323
C:\Users\sun\AppData\Local\Temp\PHP4B92.tmp:444
```
That's why I added the system's temporary directory to the blacklist instead.

Before doing this change, I considered two alternatives:

1. Changing PHPUnit_Util_Filter to remove the last stack frames, if their file paths point to the temp directory.

2. Adjusting _all_ PHPT tests to include an `%A` where stack traces are asserted in the output, so as to account for the two additional frames on Windows.

(2) would be cumbersome to maintain + tests would be guaranteed to break on Windows in the future again.  (1) sounds nice, but requires relatively complex code to introspect each stack trace.

Therefore, I went with this KISS solution.