chore: write shell script for running swiftui tests from terminal

[russellwheatley]

Hey there! So you want to contribute to FirebaseUI? Before you file this pull request, follow these steps:

• Read the contribution guidelines.
• If this has been discussed in an issue, make sure to mention the issue number here. If not, go file an issue about this to make sure this is a desirable change.
• If this is a new feature please co-ordinate with someone on FirebaseUI-Android to make sure that we can implement this on both platforms and maintain feature parity.

[gemini-code-assist]

Code Review

This pull request introduces a new bash script, swiftui-tests.sh, designed to run SwiftUI Auth unit, integration, and UI tests locally, mirroring the GitHub Actions workflow. It also updates .gitignore to exclude test logs and results, and adds documentation to CONTRIBUTING.md. Review feedback suggests improving the script's flexibility and performance by using a more widely available default simulator device, reducing the wait time for the Firebase emulator to start, and allowing the Firebase project ID to be configured via an environment variable.