tests: "make check-system" uses Guix built with (guix self).

tests: "make check-system" uses Guix built with (guix self).

This is a followup to 7e6d8d366a61f951936ed83371877ce006f679f6.
It means that "make check-system" can run tests from (gnu tests install)
much faster because it does not need to build a full 'guix' package.

* gnu/ci.scm (channel-instance->package): Export.
* build-aux/run-system-tests.scm (tests-for-channel-instance): New
procedure, with code formerly in 'run-system-tests'.
(run-system-tests): Call 'interned-file' for SOURCE, and add call to
'tests-for-channel-instance'.