Tests: Explicitly allow usage of git file-based protocol in test cases

Due to CVE-2022-39253, Git 2.30.6 sets `protocol.file.allow=user` by
default.  The change has also been backported to other Git versions by
distros.  This breaks some of our test cases that use the file-based
protocol locally to simulate real workflows without requiring network
access.  In these cases the file protocol is safe, so explicitly enable
it in the tests.