(#14600) Fix cleanup of tempfiles in file_spec

The file integration test uses temporary files that will be removed
after each test. A few tests however use the backup functionality of the
file resource type and these backups were not removed.

Create a temporary directory where the dummyfile is placed. This way the
spec helper will remove the whole directory after each test including
every backup of the dummyfile that might have been created during the
testrun.