GIMP 2.6 session files can't have many docks in a dock window, so
remove that from the GIMP 2.6 session file we use for compatibility
testing. Also remove the "gimp-single-image-window" entry.
The downside is that we need a separate 'expected'-file, but we need
to live with that.
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.
