unix: don't fail TestPoll on EINTR from Poll

Occassionaly TestPoll fails on some builders due to Poll getting
interrupted:

--- FAIL: TestPoll (0.00s)
    syscall_unix_test.go:516: Poll: unexpected error: interrupted system call
FAIL
FAIL	golang.org/x/sys/unix	0.956s

Fix this by retrying Poll in case of EINTR, same as CL 207861 did in
TestSelect.

Change-Id: I78ada14d7409e6cde852d76abd5a9a1b1bb645d2
Reviewed-on: https://go-review.googlesource.com/c/sys/+/298189
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Trust: Matt Layher <mdlayher@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matt Layher <mdlayher@gmail.com>

unix/syscall_unix_test.go

@@ -509,16 +509,21 @@ func TestPoll(t *testing.T) {
 		}
 	}()
 
-	fds := []unix.PollFd{{Fd: int32(f.Fd()), Events: unix.POLLIN}}
-	n, err := unix.Poll(fds, timeout)
-	ok <- true
-	if err != nil {
-		t.Errorf("Poll: unexpected error: %v", err)
-		return
-	}
-	if n != 0 {
-		t.Errorf("Poll: wrong number of events: got %v, expected %v", n, 0)
-		return
+	for {
+		fds := []unix.PollFd{{Fd: int32(f.Fd()), Events: unix.POLLIN}}
+		n, err := unix.Poll(fds, timeout)
+		ok <- true
+		if err == unix.EINTR {
+			t.Logf("Poll interrupted")
+			continue
+		} else if err != nil {
+			t.Errorf("Poll: unexpected error: %v", err)
+			return
+		}
+		if n != 0 {
+			t.Errorf("Poll: wrong number of events: got %v, expected %v", n, 0)
+		}
+		break
 	}
 }