For others listening in, the solution that should work (which was posted to the go-nuts list) is to "touch" the timestamps of all the "*.a" files under go_appengine/goroot.
$ cd go_appengine/goroot
$ find . -name "*.a" -exec touch {} \;
This should update the timestamps of all the "*.a" files under "goroot" which should prevent "goapp test" from attempting to rebuild packages that were already built.
After this, try your "goapp test" command again.
Somehow all the timestamps in your distribution got messed up. You might try re-installing directly from the SDK. In my experience, some versions of "unzip" (like those provided by cygwin) get the timestamps muddled. You might also try a different version of unzip, like the excellent "7z.exe" utility on Windows.
-- Glenn