shell: new `fail_while_matching` and `succeed_while_matching`.

* specs/shell_spec.yaml: Specify behaviour of new matchers.
* lib/specl/shell.lua (matchers.fail_while_matching)
(matchers.succeed_while_matching): Implement new behaviours.
* NEWS: Update.

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