[TASK] Activate all packages in unit tests

Introduce a new package manager class for unit tests
that does not read the PackageState.php file any more
but scans for packages and activates them,
so that the class loader can find the classes of all
extensions even if the according extension is not active in
the current TYPO3 installation.

Since the unit test bootstrap does not read other
extension files (ext_localconf.php, etc.) any more,
we can be sure that this does not introduce any
further side effects.

Since the PackageStates.php file will then be obsolete,
it will be removed from the travis integration repository.

Resolves: #57487
Releases: 6.2
Change-Id: Id0cd50db68fd94a31a996496fd5c54049ca94e78
Reviewed-on: https://review.typo3.org/29036
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel