Make our test cases write gimpdir files in a dedicate output directory
by changing GIMP2_DIRECTORY at runtime. This ends the writing to the
source directory which make distcheck doesn't like, and it also allows
us to have a separate expected sessionrc for the session management
test. In that test we load a GIMP 2.6 formated sessionrc but write one
updated for 2.8.

We perform this change by passing the builddir through an environment
variable to the test cases. The patch also does some generalizations
in gimp-app-test-utils.c and adapts test cases accordingly.

Also do an adjustment in sessionrc on the toolbox size since
GIMP/GTK+/X/whatever has a tendency to adjust that during runs. The
new size is stable for the moment.