The manifest can be passed to 'guix build -m', 'guix weather -m', and so
on. It can also be passed to an installed 'guix' (without
./pre-inst-env), with the exception so far of installation tests.
* build-aux/run-system-tests.scm: Remove. Move interesting bits move
to...
* etc/system-tests.scm: ... here. New file.
* Makefile.am (EXTRA_DIST): Remove 'build-aux/run-system-tests.scm' and
add 'etc/system-tests.scm'.
(check-system): Rewrite to run 'guix build -m etc/system-tests.scm'.
