addpref.m: Recode BIST test to work with multiple running instances of Octave. When running multiple instances of the Octave test suit (such as on the build bots) there can be conflict over the file /tmp/.octave_prefs. Avoid that conflict by having a per-Octave-instance directory. * addpref.m: Use tempname() and mkdir() to create a per-Octave-instance directory used for BIST test. Delete the temporary directory at the end of the test.