formatters: don't repeat contexts on every example.

The previous fix printed previously elided contexts too often.
* specs/formatter/report_spec.yaml
(does not repeat contexts consecutively): Specify a single output
for a non-filtered context with multiple nested examples.
* lib/specl/formatter/report.lua (show_contexts): Implement new
behaviour.

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