[Fuchsia] Add Netstack2 test bots

These run tests against Fuchsia's Netstack2 because the default netstack
will be moving to Netstack3 soon, but we need to maintain test coverage
until Netstack2 is no longer running anywhere.

Bug: 370083257
Change-Id: Ifc4d7edd3ebfa9d7cb87a91dbd90181ed5c6ed26
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6330942
Reviewed-by: Zijie He <zijiehe@google.com>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432212}