net: add missing error check in test

Change-Id: Id2e57bc8e18e062f60c6ac8a58dc15e049352088
GitHub-Last-Rev: 6d33b809cf8a3b5412333b1c3cc237000be8101d
GitHub-Pull-Request: golang/go#30016
Reviewed-on: https://go-review.googlesource.com/c/160440
Run-TryBot: Mikio Hara <mikioh.public.networking@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Mikio Hara <mikioh.public.networking@gmail.com>

src/net/dial_test.go

@@ -338,6 +338,11 @@ func TestDialParallel(t *testing.T) {
 		if c != nil {
 			c.Close()
 		}
+		if tt.expectOk && err != nil {
+			t.Errorf("#%d (cancel): got %v; want nil", i, err)
+		} else if !tt.expectOk && err == nil {
+			t.Errorf("#%d (cancel): got nil; want non-nil", i)
+		}
 		elapsed = time.Now().Sub(startTime)
 		if elapsed > 100*time.Millisecond {
 			t.Errorf("#%d (cancel): got %v; want <= 100ms", i, elapsed)