fix: submit form with external submit button when user clicks enter (#1220)

[struank-l]

What

This PR fixes bug #1220 - userEvent.keyboard('{Enter}') doesn't trigger form submission when the submit button is outside the form element, even if it’s associated with the form element by using the button's form attribute.

Why

form should be submitted if the user clicks enter on a field in the form and there is a button outwith the form linked by form attribute.

How

if a submit button is not found within the form by the current code, and the form has an id set then search within the document for a suitable button with the correct form id

Checklist

• n/a Documentation
• [ x] Tests
• [ x] Ready to be merged

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.