Simpletest fails to run PHPUnit on Windows and in extracted tarballs of Drupal

Follow-up to #2114823: Update PHPUnit to 4.x

The update to PHPUnit 4 moved the phpunit executable to a different location:

However, the update was performed on a Linux platform, so the phpunit executable installed by Composer is not the executable for Windows.

The above is just a *nix link to phpunit's main shell script:

...which also happens to work on Windows.

Attached patch

1. fixes the phpunit script command location
2. removes the error suppression for reading the JUnit XML result file, as it only hides an early error condition that blows up later on anyway.