Reland "[fuchsia] Enable UDPSocketTest.JoinMulticastGroup"

This is a reland of 1bef8501

Original change's description:
> [fuchsia] Enable UDPSocketTest.JoinMulticastGroup
>
> Assert after each send call to better pinpoint future problems.
>
> The test passes locally.
>
> Fixed: 1125855
> Change-Id: I30d5c7bd2bc14fbd9547b1eb6d309d7556274891
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404483
> Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
> Reviewed-by: Wez <wez@chromium.org>
> Commit-Queue: Tamir Duberstein <tamird@google.com>
> Cr-Commit-Position: refs/heads/master@{#806111}

Fixed: 1127298
Change-Id: I8d20c8247d9729511963eff35a6f276e0a82ae78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405934Reviewed-by: Wez <wez@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Tamir Duberstein <tamird@google.com>
Cr-Commit-Position: refs/heads/master@{#831927}

testing/buildbot/filters/fuchsia.services_unittests.filter

 # https://crbug.com/1125854 - IsTracingEnabled() expectations not met.
 -TracingConsumerTest.NotifiesOnTracingEnabledWaitsFor*
-
-# https://crbug.com/1125855 - Second socket receives too many packets.
--UDPSocketTest.JoinMulticastGroup