test: fix flaky TestAdd_Close_concurrent

We need to reset an error to pool.ErrClosed in a Pool.Add()
method if a pool already closed to make the behavior
deterministic.

pool/connection_pool.go

@@ -283,6 +283,12 @@ func (p *ConnectionPool) Add(ctx context.Context, instance Instance) error {
 			canceled = false
 		}
 		if canceled {
+			if p.state.get() != connectedState {
+				// If it is cancelled due to a Close()/CloseGraceful() call we
+				// overwrite the error to make behavior expected.
+				err = ErrClosed
+			}
+
 			p.endsMutex.Lock()
 			delete(p.ends, instance.Name)
 			p.endsMutex.Unlock()