net: re-enable TestVariousDeadlines on Plan 9

net: re-enable TestVariousDeadlines on Plan 9

After CL 470215 it should be safe to run this test on Plan 9.

Fixes #26945

Change-Id: I17880b696d2e91e6c4862fac05e6372f44e69b22
Reviewed-on: https://go-review.googlesource.com/c/go/+/472436
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>