wvtest.py: correct bup-python in #! preamble
wvtest.py: correct bup-python in #! preamble

Otherwise, manual invocations like "./wvtest.py lib/bup/t/tgit.py" fail.

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

tjanez/bup,gevaerts/bup,tjanez/bup,tjanez/bup,gevaerts/bup,ToxicFrog/bup,ToxicFrog/bup,ToxicFrog/bup,tjanez/bup,ToxicFrog/bup,tjanez/bup,gevaerts/bup,ToxicFrog/bup,tjanez/bup,gevaerts/bup,gevaerts/bup