os: enable TestClosedPipeRace* on FreeBSD

This test has worked since CL 165801 (committed March 12, 2019), so
stop skipping it. With this, we check that Close makes concurrent I/O
operations on pipes return Errclosed on all platforms.

Updates #19093.

Change-Id: Ic090c70996c115abf80d8f9b93ca2aeaf347c9d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/371016
Trust: Austin Clements <austin@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/os/pipe_test.go

@@ -150,11 +150,6 @@ func TestStdPipeHelper(t *testing.T) {
 }
 
 func testClosedPipeRace(t *testing.T, read bool) {
-	switch runtime.GOOS {
-	case "freebsd":
-		t.Skip("FreeBSD does not use the poller; issue 19093")
-	}
-
 	limit := 1
 	if !read {
 		// Get the amount we have to write to overload a pipe