$_ is clobbered by debug trap

[HaleTom]

bats suffers from the same bug resolved in:

• https://github.com/postmodern/chruby/pull/381
• https://github.com/rcaloras/bash-preexec/pull/32

This is a "feature" of bash, see this answer on SO.

I discovered this when my bash-preexec returned a false-positive due to this "feature" not being worked-around in bats.