Changes

tests: sanitize all cards before running tests

Even though our tests are written in a way that they shouldn't interfere
with each other, it may happen that one testrun writes content to a file
that upsets a different testrun. The resulting problems are often
difficult to diagnose.

To minimize the problem, let's reset the cards to a defined state before
running the actual tests. This can be done using pySim-shell's export
feature. We can generate a backup from a known good state and then play
back the backup to reset files that have been changed. Files that didn't
change will not be written thanks to the conserve_write feature of
pySim-shell.

Related: OS#4384
Change-Id: I42eaf61280968518164f2280245136fd30a603ce