[BUGFIX] Incomplete mock in ext:install

"ExtensionCompatibilityTesterTests" triggers loadBaseTca() that
loads TCA and TCA overrides of all active packages. Since the
unit test package manager activates all extensions found, this
may lead to unexpected effects with extensions hanging around
in typo3conf/ext.
The patch creates a backup of the package manager and mocks it
properly in some tests.

Change-Id: Ia5b1ccb449f1611837e6436cab226820b22a9047
Resolves: #60120
Related: #59770
Releases: 6.3, 6.2
Reviewed-on: https://review.typo3.org/31362
Reviewed-by: Markus Klein
Tested-by: Markus Klein