Rigid build time limits let some unit tests fail on slow hardware.

Set default timeout to 5 minutes and make it configurable via
`GUIX_TESTS_BUILD_TIMEOUT` environment variable.

* guix/tests.scm (%tests-build-timeout): New variable.
(open-connection-for-tests, call-with-external-store): Use it.
* tests/guix-environment-container.sh, tests/guix-home.sh,
tests/guix-pack-localstatedir.sh, tests/guix-pack-relocatable.sh: Use it to
define ‘GUIX_BUILD_OPTIONS’.

Change-Id: Iab63c2dcf4a3ad311a0cf2f39d7ea2ec158ddbb4
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

This ensures ‘make check’ doesn’t accidentally start building the world.

* tests/guix-environment-container.sh (GUIX_BUILD_OPTIONS): New environment
variable.
* tests/guix-home.sh (GUIX_BUILD_OPTIONS): Likewise.
* tests/guix-pack-localstatedir.sh (GUIX_BUILD_OPTIONS): Likewise.
* tests/guix-pack-relocatable.sh (GUIX_BUILD_OPTIONS): Likewise.

Change-Id: If8afe9cfc5c0f1f72cd6f3d39dd5d4f6415d1e02

Fixes <https://bugs.gnu.org/35776>.
Reported by Ting-Wei Lan <lantw44@gmail.com>.

* tests/guix-pack-localstatedir.sh: Set 'storedir' before
'NIX_STORE_DIR'.
* tests/guix-pack-relocatable.sh: Likewise.

* guix/scripts/pack.scm (self-contained-tarball): Add #:profile-name and
honor it.
(squashfs-image, docker-image): Add #:profile-name.
(%default-options): Add 'profile-name'.
(%options, show-help): Add "--profile-name".
(guix-pack): Honor it.
* tests/guix-pack-localstatedir.sh: New file.
* Makefile.am (SH_TESTS): Add it.
* doc/guix.texi (Invoking guix pack): Document "--profile-name".