specs: elide clone_rename deprecation warning, with Specl 12.

Fix #54.
* specs/spec_helper.lua.in (capture): Stub inprocess.capture if
installed Specl is too old to implement it.
* specs/table_spec.yaml (clone_rename): Wrap clone_rename in
inprocess.capture to elide deprecation warning on stderr.

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