Prepare for having multiple tests in TestGetAllCredentials
Prepare for having multiple tests in TestGetAllCredentials

Just turn the existing test case into an array, and re-create
the test file from scratch in each iteration.

Also ensure the temporary file is removed eventually.

Signed-off-by: Miloslav Trmač <9c008e38982a5397deb855345fb164f0558459ae@redhat.com>

containers/image,containers/image