System test manifest

["Ludovic Courtès" <ludo@gnu.org>]

Hello Guix!

Commit 5ec4156bbcaec8337f78411204d59e59e706103b adds a manifest for
system tests.  Now “make check-system” simply does:

  ./pre-inst-env guix build -m etc/system-tests.scm

The nice thing is that it’s more flexible than the custom script we had
since it gives access to all the command-line options.

As a bonus (this is directly inspired by discussions in the “release”
working group at the Guix Days), you can pass it to ‘guix weather’ to
get test coverage data.  There are probably a couple of things to
improve in ‘guix weather’ to make it more convenient, such as adding a
flag to list missing substitutes.

My next goal is to have a manifest for “release-critical things” that
one can again pass to ‘guix build’ or ‘guix weather’ to have an
immediate picture of the “releasability” status.

Feedback welcome!

Ludo’.