Add a 'make stupid' target that does 'make test' with a minimal PATH.
Add a 'make stupid' target that does 'make test' with a minimal PATH.

Because I'm stupid and I keep forgetting to test what happens if you don't
have 'bup' in your PATH.

Thanks to Dave Coombs and Andy Chong for reporting the problem.  And in
v0.01 too.

jbaber/bup,pombredanne/bup,apenwarr/bup,apenwarr/bup,apenwarr/bup,tjanez/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,ToxicFrog/bup,mhoeher/bup,gevaerts/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,tjanez/bup,ToxicFrog/bup,pombredanne/bup,tjanez/bup,mhoeher/bup,gevaerts/bup,jbaber/bup,pombredanne/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,pombredanne/bup,gevaerts/bup,tjanez/bup,gevaerts/bup,pombredanne/bup,gevaerts/bup,mhoeher/bup,apenwarr/bup,jbaber/bup,ToxicFrog/bup,tjanez/bup,tjanez/bup