Minor alignment in $root when ResourceManager instantiated with classloader

There was an inconsistency in the value of $root in ResourceManager when it has constructed with a string path or a ClassLoader object. It was interesting just in case when Bolt is created by Bolt\Configuration\Composer. Now $root points the root path of the whole project in both cases.
By this change I had to touch unit tests and reverted them manually to a previous state. They are now initialized with TEST_ROOT as earlier.