chore(tests): improve unit test output

[daniel-sanche]

In #18122, the tests would stall without any logs after Running tests across 1 package(s) using 1 parallel worker(s)...

This is because the unit test job would wait until tests were complete before printing outputs, which isn't helpful when a job freezes

This PR attempts to improve the usability of unit tests by:

• showing live logs when PARALLEL_WORKERS is 1
  • this is always the case for unit tests, but mypy runs in parallel
  • parallel jobs still buffer their writes, to avoid overlapping logs
• fail-fast. If one nox session or package fails, the CI worker exits with an error immediately
• use emojis to designate pass/fail (✅/❌), to make it logs to scan
• added --no-tags, to avoid unneeded download and printing

[gemini-code-assist]

Code Review

This pull request introduces real-time log streaming in the CI test runner when running with a single worker, while maintaining buffered logs for parallel execution. It also adds a global autouse fixture to mock the MetricServiceClient in unit tests and ensures proper cleanup of GoogleCloudMetricsHandler instances using try-finally blocks. Feedback is provided regarding a potential bash error in the test runner script when checking the PARALLEL_WORKERS variable if it is unset or empty.