app/tests: Add test for GIMP 2.8 multi window session files
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.
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