feat: Signup screen widget test

Description

[x] ✨ New feature (non-breaking change which adds functionality)
[ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
[x] ❌ Breaking change (fix or feature that would cause existing functionality to change)
[ ] 🧹 Code refactor
[ ] ✅ Build configuration change
[ ] 📝 Documentation
[ ] 🗑️ Chore

[ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[ ] I read and ran all relevant commands as specififed in the Running Tests section of the Contributor Guide.
[ ] The title of the PR follows the Conventional Commits guideline
[ ] My local branch follows the naming standards in the Deepsource Branch Naming Convention or Biodiversity Branch Naming Convention
[ ] I listed at least one issue that this PR fixes in the description above.
[ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy,
[ ] I updated/added relevant documentation (doc comments with ///).
[ ] I added new tests to check the change I am making, or this PR is [test-exempt].
[ ] All existing and new tests are passing.