Move code in test-session-management.c to a common place so we can
have several variants of session management tests. We can't easily run
all session management tests in a single process, because some parts
of Gimp (the GObject) initialization eeks if done twice in the same
process.

Rename test-session-management.c to test-session-2-6-compatibility.c
and create a new test for GIMP 2.8 multi-window mode session files,
namely test-session-2-8-compatibility-multi-window.c. Copy and rename
sessionrc files accordingly.