The systemd package has a few tests for udev. The ebuild should run those.
soap added tests in https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=eb6b84e59fabdec573a61ea845edc144541c6b75. See also: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=ebee474c08d6219b0a0ff88139ce0aabf3ec529c https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=4e3700c8ee0660cc8e4a97d194592905bbe852a9 https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=16e67b51511f6ad9cdffa1f1bf9ec3b109dd645c