[tests] Use static networking in test VM; restructure test harness.

[tests] Use static networking in test VM; restructure test harness.

QEMU user-mode networking provides DHCP, but dhcpcd can hang during
early boot when the DHCP server is slow to respond, blocking the
marionette REPL from connecting.  Replace it with static networking
(10.0.2.15/24 on eth0, gateway 10.0.2.2).

system-test-runner exits the process after the first test group
completes, so the VPS test must consolidate all assertions into a
single test-begin/test-end wrapping.  Each test module (sshd,
networking, ntp) now returns bare assertions without its own
begin/end pair.