refactor: remove redundant environment creation.

* specs/environment_spec.yaml: New examples to ensure before and
after blocks don't have access to each others' environments.
* lib/specl/runner.lua (run_examples): Remove redundant nested
environment creation.  The environment shared by an example
block is sufficient, and must be shared by the before and after
blocks of each example anyway.

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