[network] Unbreak guest tests a little bit

Machina networking is disabled (see https://fxbug.dev/410037697). But
these tests are currently broken because of log initialization.

Fix a little bit of that so we actually hang on the broken Machina state
instead.

Change-Id: I974e110272acf7b35a947cff55f866246a6a6964
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1442593
Reviewed-by: Zeling Feng <[email protected]>
Commit-Queue: Bruno Dal Bo <[email protected]>
Fuchsia-Auto-Submit: Bruno Dal Bo <[email protected]>

• src/connectivity/network/tests/integration/guest/dhcpv4/BUILD.gn[diff]
• src/connectivity/network/tests/integration/guest/dhcpv4/src/lib.rs[diff]
• src/connectivity/network/tests/integration/guest/dhcpv6/BUILD.gn[diff]
• src/connectivity/network/tests/integration/guest/dhcpv6/src/lib.rs[diff]