tests: Add tests for `components/ProposalForm.tsx`

@RossaMania got a great start on this in [unit-tests/testPropose.astro.ts](https://github.com/tulsawebdevs/website/blob/9c5b753cd0013def87a0ddb803054fd63f8b8720/src/unit-tests/testPropose.astro.ts) (we'll want to update the name to match the name of the file under test).