I do not think it is always Mother who "tests", but rather it is the nature of human life. Didn't Mother say that even the Incarnation of God must suffer because of human birth?

yet Mother assures us that She is with us always. Where else will she be? Think this: A Mother like mine does not hide! She will not abandon her own child!

I think it is we who have doubts, and thus do not see Her. Even this, Mother assures us, is natural. Doubts are the buds of our deepening faith.