[task/fix-test-autoloader] Fix autoloader to support failing/erroring te...

...sts.

Before showing error output PHPUnit 3.7.x calls class_exists() on some PHPUnit
Extension class names that may not exist. Calling class_exists() already
triggers the autoload function in which require() then obviously fails.

We now check whether a file is includable by simply looping over all possible
include directories.