refactor: rationalise and normalise shell matchers.

* lib/specl/shell.lua (contain_output, match_output)
(contain_error, match_error): Rename from these...
(output_containing, output_matching, output_error_containing)
(output_error_matching): ...to these.
Adjust all callers.
* NEWS: Update.

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