Merge pull request #13585 (nixos-tests-splitup)

This splits a few NixOS tests (namely Chromium, VirtualBox and the
networking tests) into several subtests that are exposed via attributes.

The networking tests were already split up but they didn't expose an
attribute set of available tests but used a function attribute to
specify the resulting test instead.

A new function callSubTests in nixos/release.nix is now responsible for
gathering subtests, which is also used for the installer and boot tests.
The latter is now placed in a tests.boot.* namespace rather than
"polluting" the tests attribute set with its subtest.