refactor: move common spec initialisation to spec_helper.lua.

This also allows running the latest (unreleased) specl directly
from the bulid directory without having to call `make check`.
* specs/spec_helper.lua: New file.  Set package.path and
package.cpath so that specl can find specified modules without
LUA_PATH and LUA_CPATH.
Add common code...
* specs/lposix_spec.yaml, specs/posix_spec.yaml: ...from here.
* specs/specs.mk (EXTRA_DIST): Add specs/spec_helper.lua.

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