* gnu/tests/audio.scm, gnu/tests/avahi.scm, gnu/tests/ci.scm,
gnu/tests/containers.scm, gnu/tests/cups.scm, gnu/tests/databases.scm,
gnu/tests/dict.scm, gnu/tests/dns.scm, gnu/tests/docker.scm,
gnu/tests/file-sharing.scm, gnu/tests/guix.scm, gnu/tests/install.scm,
gnu/tests/ldap.scm, gnu/tests/lightdm.scm, gnu/tests/mail.scm,
gnu/tests/messaging.scm, gnu/tests/monitoring.scm, gnu/tests/networking.scm,
gnu/tests/nfs.scm, gnu/tests/package-management.scm, gnu/tests/rsync.scm,
gnu/tests/samba.scm, gnu/tests/ssh.scm, gnu/tests/telephony.scm,
gnu/tests/upnp.scm, gnu/tests/version-control.scm,
gnu/tests/virtualization.scm, gnu/tests/vnstat.scm,
gnu/tests/web.scm: (services): Replace dhcp-client-service-type with its
replacement dhcpcd-service-type.

Change-Id: I01d7f9a8c01736d8c8da591aaf4749bbfa4dcdad

* gnu/tests/package-management.scm: Remove them.

The <nix/config.nix> support has been removed in Nix 2.4:
https://github.com/NixOS/nix/pull/4025. Rely on the store ping command instead
to check that we can communicate with the daemon.

* gnu/tests/package-management.scm (run-nix-test): Use the store ping command
instead of building a dummy derivation.

* gnu/tests/audio.scm, gnu/tests/base.scm, gnu/tests/ci.scm,
gnu/tests/cups.scm, gnu/tests/databases.scm, gnu/tests/desktop.scm,
gnu/tests/dict.scm, gnu/tests/docker.scm, gnu/tests/file-sharing.scm,
gnu/tests/ganeti.scm, gnu/tests/guix.scm, gnu/tests/ldap.scm,
gnu/tests/linux-modules.scm,
gnu/tests/mail.scm, gnu/tests/messaging.scm, gnu/tests/monitoring.scm,
gnu/tests/networking.scm, gnu/tests/nfs.scm,
gnu/tests/package-management.scm, gnu/tests/reconfigure.scm,
gnu/tests/rsync.scm, gnu/tests/security-token.scm,
gnu/tests/singularity.scm, gnu/tests/ssh.scm, gnu/tests/telephony.scm,
gnu/tests/version-control.scm, gnu/tests/virtualization.scm,
gnu/tests/web.scm: Remove (mkdir #$output) (chdir #$output) and
pass #$output as argument to 'system-test-runner'.

In Guile 3.0.7, 'test-runner-current' is set to #f upon 'test-end'.
Consequently, the previous strategy, where we'd call
'test-runner-current' after 'test-end', no longer works.  Instead, set
the test runner in each test right before 'test-begin'.

* gnu/build/marionette.scm (system-test-runner): New procedure.
* gnu/tests/audio.scm (run-mpd-test): Replace (exit (= ...)) idiom
by (test-runner-current (system-test-runner)).
* gnu/tests/base.scm (run-basic-test)
(run-cleanup-test, run-mcron-test, run-nss-mdns-test): Likewise.
* gnu/tests/ci.scm (run-laminar-test): Likewise.
* gnu/tests/cups.scm (run-cups-test): Likewise.
* gnu/tests/databases.scm (run-memcached-test)
(run-postgresql-test, run-mysql-test): Likewise.
* gnu/tests/desktop.scm (run-elogind-test): Likewise.
* gnu/tests/dict.scm (run-dicod-test): Likewise.
* gnu/tests/docker.scm (run-docker-test): Likewise.
(run-docker-system-test): Likewise.
* gnu/tests/file-sharing.scm (run-transmission-daemon-test): Likewise.
* gnu/tests/ganeti.scm (run-ganeti-test): Likewise.
* gnu/tests/guix.scm (run-guix-build-coordinator-test): Likewise.
(run-guix-data-service-test): Likewise.
* gnu/tests/ldap.scm (run-ldap-test): Likewise.
* gnu/tests/linux-modules.scm (run-loadable-kernel-modules-test-base): Likewise.
* gnu/tests/mail.scm (run-opensmtpd-test)
(run-exim-test, run-dovecot-test, run-getmail-test): Likewise.
* gnu/tests/messaging.scm (run-xmpp-test)
(run-bitlbee-test, run-quassel-test): Likewise.
* gnu/tests/monitoring.scm (run-prometheus-node-exporter-server-test)
(run-zabbix-server-test): Likewise.
* gnu/tests/networking.scm (run-inetd-test, run-openvswitch-test)
(run-dhcpd-test, run-tor-test, run-iptables-test, run-ipfs-test): Likewise.
* gnu/tests/nfs.scm (run-nfs-test)
(run-nfs-server-test, run-nfs-root-fs-test): Likewise.
* gnu/tests/package-management.scm (run-nix-test): Likewise.
* gnu/tests/reconfigure.scm (run-switch-to-system-test)
(run-upgrade-services-test, run-install-bootloader-test): Likewise.
* gnu/tests/rsync.scm (run-rsync-test): Likewise.
* gnu/tests/security-token.scm (run-pcscd-test): Likewise.
* gnu/tests/singularity.scm (run-singularity-test): Likewise.
* gnu/tests/ssh.scm (run-ssh-test): Likewise.
* gnu/tests/telephony.scm (run-jami-test): Likewise.
* gnu/tests/version-control.scm (run-cgit-test): Likewise.
(run-git-http-test, run-gitolite-test, run-gitile-test): Likewise.
* gnu/tests/virtualization.scm (run-libvirt-test, run-childhurd-test): Likewise.
* gnu/tests/web.scm (run-webserver-test, run-php-fpm-test)
(run-hpcguix-web-server-test, run-tailon-test, run-patchwork-test): Likewise.

* gnu/tests/package-management.scm: New file.
* gnu/local.mk: Add this.
* gnu/services/nix.scm (<nix-configuration>): New record.
(nix-activation): Generate Nix config file which fixes sandbox.
(nix-service-type): Add default value.
(nix-shepherd-service): Allow provide Nix package.
* doc/guix.texi (Miscellaneous Services)[Nix service]<nix-configuration>:
Document record.