add preferences:default-set?
fix no-write-and-frame-leak.rkt test case

more precisely, fix the way that the drracket test utilities
deal with the test suite so that when certain prefs are
initialized (as part of a test suite startup of drracket),
they actually get initialized properly (this was broken for
preferences whose value gets read particularly early)

Then use that to disable autosaving in the no-write-and-frame-leak.rkt
test