misc/cgo/test: ignore "too many open files" in issue 18146 test

Seen on the OpenBSD/AMD64 builder:
https://build.golang.org/log/fa34df1bcd3af12d4fc0fb0e60e3c6197a2a6f75

Update #18146.

Change-Id: I2646621488be84d50f47c312baa0817c72e3c058
Reviewed-on: https://go-review.googlesource.com/33907
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/cgo/test/issue18146.go

@@ -67,7 +67,7 @@ func test18146(t *testing.T) {
 			if pe, ok := err.(*os.PathError); ok {
 				err = pe.Err
 			}
-			if se, ok := err.(syscall.Errno); ok && se == syscall.EAGAIN {
+			if se, ok := err.(syscall.Errno); ok && (se == syscall.EAGAIN || se == syscall.EMFILE) {
 				time.Sleep(time.Millisecond)
 				continue
 			}