[BUGFIX] impexp: Wrong FAL references after import

Fix a case during ext:impexp FAL uid mapping where
sys_file_reference end up pointing to wrong sys_file
records in case new sys_file uids also already exist
in import file, due to multiple mappings.
Power the patch with a functional test - it fails
without the change in Import.php and is ok with the
change.
See the test method comments for more details
on what exactly is checked here.

Change-Id: I472cad2652e8d6f5d30e4ebc68ff1ae93de71ea1
Resolves: #67188
Releases: master, 8.7, 7.6
Reviewed-on: https://review.typo3.org/55368


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>