[functions] Initial testing setup

[dackers86]

Establish the testing infrastructure for the Dart Firebase Functions SDK. This includes:

• Unit test framework setup
• Integration test framework setup (using Firebase CLI)
• Test utilities and helpers
• Test coverage configuration

This ensures the SDK can be thoroughly tested and validated.

Dependencies:

• [functions] Repository setup
• [functions] SDK scaffolding

Acceptance Criteria:

• Unit test framework configured
• Integration test framework configured (using Firebase CLI)
• Test utilities and helpers created
• Test coverage reporting set up