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 <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: millerresearch

src/net/timeout_test.go

@@ -909,10 +909,6 @@ func TestVariousDeadlines4Proc(t *testing.T) {
 }
 
 func testVariousDeadlines(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping test on plan9; see golang.org/issue/26945")
-	}
-
 	handler := func(ls *localServer, ln Listener) {
 		for {
 			c, err := ln.Accept()