wgengine/magicsock: add disabled failing (deadlocking) test for #1282

The fix can make this test run unconditionally.

This moves code from 5c619882bc for
testability but doesn't fix it yet. The #1282 problem remains (when I
wrote its wake-up mechanism, I forgot there were N DERP readers
funneling into 1 UDP reader, and the code just isn't correct at all
for that case).

Also factor out some test helper code from BenchmarkReceiveFrom.

The refactoring in magicsock.go for testability should have no
behavior change.

wgengine/magicsock/magicsock.go

@@ -1445,28 +1445,40 @@ func (c *Conn) runDerpReader(ctx context.Context, derpFakeAddr netaddr.IPPort, d
 			continue
 
 		}
-		// Before we wake up ReceiveIPv4 with SetReadDeadline,
-		// note that a DERP packet has arrived.  ReceiveIPv4
-		// will read this field to note that its UDP read
-		// error is due to us.
-		atomic.AddInt64(&c.derpRecvCountAtomic, 1)
-		// Cancel the pconn read goroutine.
-		c.pconn4.SetReadDeadline(aLongTimeAgo)
 
+		if !c.sendDerpReadResult(ctx, res) {
+			return
+		}
 		select {
 		case <-ctx.Done():
 			return
-		case c.derpRecvCh <- res:
-			select {
-			case <-ctx.Done():
-				return
-			case <-didCopy:
-				continue
-			}
+		case <-didCopy:
+			continue
 		}
 	}
 }
 
+// sendDerpReadResult sends res to c.derpRecvCh and reports whether it
+// was sent. (It reports false if ctx was done first.)
+//
+// This includes doing the whole wake-up dance to interrupt
+// ReceiveIPv4's blocking UDP read.
+func (c *Conn) sendDerpReadResult(ctx context.Context, res derpReadResult) (sent bool) {
+	// Before we wake up ReceiveIPv4 with SetReadDeadline,
+	// note that a DERP packet has arrived.  ReceiveIPv4
+	// will read this field to note that its UDP read
+	// error is due to us.
+	atomic.AddInt64(&c.derpRecvCountAtomic, 1)
+	// Cancel the pconn read goroutine.
+	c.pconn4.SetReadDeadline(aLongTimeAgo)
+	select {
+	case <-ctx.Done():
+		return false
+	case c.derpRecvCh <- res:
+		return true
+	}
+}
+
 type derpWriteRequest struct {
 	addr   netaddr.IPPort
 	pubKey key.Public