MessagePumpEpoll: Migrate from pipe to socketpair in tests

This CL picks up the CL:5064411 to add `NestedNotification` test
case and disable by default. It also removes pipe in favor of socketpair
in tests, and clears legacy code.

CL:5064411:
> Regression test for MessagePumpLibevent/Epoll
>
> If an FD watcher runs a nested RunLoop on an FD event, and another
> watcher for the same FD cancels the outer watcher's watch from within
> the nested RunLoop, we'll UAF in MessagePumpEpoll today.
>
> This is not something we currently hit in production, but we will if
> we fully migrate MessagePumpLibevent to MessagePumpEpoll; so migration
> is blocked on this bug.
>
> This is a regression test for the bug. Namely:
>
>    MessagePumpLibeventTest.NestedNotification/1
>
> should crash due to this bug.
>
> Bug: 1469529
> Change-Id: I335c213437b6766aa9101fa013ed5a2e3f02ac65

Bug: 1469529
Change-Id: Ib3a36f061f3cd695533671362906353c20dfd0a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5221714
Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Francois Pierre Doray <fdoray@chromium.org>
Commit-Queue: Jianhui J Dai <jianhui.j.dai@intel.com>
Cr-Commit-Position: refs/heads/main@{#1251886}