Contact form: Send test form button

### What

Button on the form block settings above Send email to that will send a test form submission to that email.

### Why

I have received several chats of users thinking the form block is broken because they fill out the form with "test" or gibberish "ifhiuwahgeuwg4y." Our system flags this as spam and the form email is never sent so they assume there is an error. By adding this button we can allow them to easily test and confirm the email so they don't feel the need to come into chat to check on something due to a misunderstanding.

### How

Just a simple button that sends default content for each block in the form block that they have used.

No files, tests, or entry points are named. Start by locating the form block settings and the existing Send email behavior, then trace how submissions are validated and delivered; done means a clearly defined test-submission flow that confirms delivery without being treated as spam.

Written by the indexing model from the issue text.