Dynamic added inputs not triggers form revalidation

[felixmosh]

Due to my refactor of improve initial render (#490 (comment)) there is a bug which not revalidate the form (if the input is not part of the form render path).

I've created an example which demonstrate it https://codesandbox.io/s/formsy-semantic-ui-formselect-options-issue-forked-wq32d?file=/src/Inputs.js

This happens because I've removed the validate from the form's attachToForm method, because the test case was mistakenly tried to submit the form (which triggered re-validation).