wvtest: chdir to start dir before extracting stack
wvtest: chdir to start dir before extracting stack

It looks like traceback may not be able to find the source text if the
current directory has been changed from the original.

Signed-off-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>
Tested-by: Rob Browning <1dc5917c2adeeba13d1ef33dbb40691a1b67cc65@defaultvalue.org>

ToxicFrog/bup,ToxicFrog/bup,ToxicFrog/bup,ToxicFrog/bup,ToxicFrog/bup