report: elide contexts for filtered-out examples.

* specs/formatter/report_spec.yaml: Specify output of report
formatter when some examples are filtered out.
* lib/specl/formatter/report.lua (spec): Remove.
Adjust all callers.
(show_contexts): New function to show not yet displayed context
lines for filtered-in examples.
(expectations): Use it.
* NEWS: Update.

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