unix: don't call t.Fatal from goroutine in TestSendmsgBuffers

Change-Id: Iddef23a1fb2fcde4560ec0bb9e9ae0301fef965f
Reviewed-on: https://go-review.googlesource.com/c/sys/+/429415
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

unix/syscall_unix_test.go

@@ -975,7 +975,8 @@ func TestSendmsgBuffers(t *testing.T) {
 		}
 		n, oobn, recvflags, _, err := unix.RecvmsgBuffers(fds[1], bufs, nil, 0)
 		if err != nil {
-			t.Fatal(err)
+			t.Error(err)
+			return
 		}
 		if n != 10 {
 			t.Errorf("got %d bytes, want 10", n)