os: skip TestPipeThreads on Solaris

I don't know why it is not working.  Filed issue 19111 for this.

Fixes build.

Update #19111.

Change-Id: I76f8d6aafba5951da2f3ad7d10960419cca7dd1f
Reviewed-on: https://go-review.googlesource.com/37092Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/os_test.go

@@ -1947,6 +1947,8 @@ func TestPipeThreads(t *testing.T) {
 	switch runtime.GOOS {
 	case "freebsd":
 		t.Skip("skipping on FreeBSD; issue 19093")
+	case "solaris":
+		t.Skip("skipping on Solaris; issue 19111")
 	case "windows":
 		t.Skip("skipping on Windows; issue 19098")
 	case "plan9":