specs: update custom matchers for Specl 11.

* bootstrap.conf (buildreq): Bump specl minimum version to 11.
* specs/spec_helper.lua (matchers.have_size, matchers.have_member):
Matcher object functions require an initial `self` parameter.
(matchers.fail_with): Remove.

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