os: enable TestPipeThreads on FreeBSD

This test works on FreeBSD since CL 165801 was submitted.

Updates #19093

Change-Id: I45ffeb403c1de4385cdb21b9647f21976061e1ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/373358
Trust: Tobias Klauser <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: tklauser

src/os/os_test.go

@@ -2434,8 +2434,6 @@ func TestRemoveAllRace(t *testing.T) {
 // Test that reading from a pipe doesn't use up a thread.
 func TestPipeThreads(t *testing.T) {
 	switch runtime.GOOS {
-	case "freebsd":
-		t.Skip("skipping on FreeBSD; issue 19093")
 	case "illumos", "solaris":
 		t.Skip("skipping on Solaris and illumos; issue 19111")
 	case "windows":