test: fix flaky TestAdd_Close_concurrent

An error may appear in a test if a connection to an instance was
canceled by a concurrent pool.Close() call:

```
failed to dial: dial tcp host:port: operation was canceled
```

It is expected behavior and we should to fix the test.

pool/connection_pool_test.go

@@ -676,7 +676,12 @@ func TestAdd_Close_concurrent(t *testing.T) {
 
 		err = connPool.Add(ctx, makeInstance(serv1, connOpts))
 		if err != nil {
-			assert.Equal(t, pool.ErrClosed, err)
+			// Pool should be already closed or in-progress connect is
+			// canceled.
+			if err != pool.ErrClosed {
+				// There is no constant in the `net` package for the error.
+				assert.ErrorContains(t, err, "operation was canceled")
+			}
 		}
 	}()