shell: new `fail` and `succeed` matchers.

* specs/shell_spec.yaml (describe fail matcher): Specify
behaviour of fail matcher.
(describe succeed matcher): Similarly for succeed.
* lib/specl/shell.lua (matchers.fail, matchers.succeed):
Implement newly specified behaviours.
* NEWS: Update.

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