bootstrap.php is not loaded in process isolation with $preserveGlobalState = FALSE

Resolves #797
Duplicates #1343 + #1346 for PHPUnit 4.2

The added regression test prominently exposes the problem:  If the test's custom bootstrap.php file is not loaded, then PHPUnit itself and `PHPUnit_Framework_TestCase` does not exist → Fatal error. :collision: :wink: 

Discussion in #797 debated whether this would be a backwards-incompatible change, but due to the above, and because I'm experiencing the exact same problem in Drupal tests in case process isolation is triggered, I'm considering this as a straight bug fix.

This is a bug, because PHPUnit fails to boot itself.

If you specify a `bootstrap` file in your configuration, then phpunit is instructed to load the specified file, regardless of which process.

Currently, this fundamental configuration value is ignored when a child process is spawned and `$preserveGlobalState` is `FALSE`.  Failure to include it means that e.g. your class loader won't exist.