shell: new `fail_while_containing` and `succeed_while_containing`.

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

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