Use test -aP instead of which -a in test helper
Use test -aP instead of which -a in test helper

The former is a bash builtin, latter not necessarily available. For
example, `which` is deprecated in Debian's debianutils >= 5.0.

sstephenson/rbenv,rbenv/rbenv,sstephenson/rbenv,rbenv/rbenv,sstephenson/rbenv