specs: specify progress formatter behaviours.

* specs/formatter/spec_helper.lua (run_spec): Refactor for better
flexibility in arguments to SPECL subprocess, without hard-coding
`--formatter=report`.
* specs/formatter/report_spec.yaml: Adjust all callers.
* specs/formatter/progress_spec.yaml: New specifications.
* specs/specs.mk (specl_SPECS): Add it.

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