specs: use a custom matcher to specify process status and error.

* specs/spec_helper.lua.in (matchers.fail_with): Custom matcher
to specify process non-zero exit as well as stderr content.
* specs/io_spec.yaml (io.die), specs/optparse_spec.yaml (io.die):
Use it to ensure that in addition to the correct error output,
the exit status is non-zero.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>