Improve integration test speed

[noah-thor]

This improves the integration test suite speed by running all integration tests in a single swift test invocation. We maintain the one @Suite at a time behavior by introducing a new test trait that uses a single mutex to limit inter-suite parallelism.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

This is meant to improve the CI/CD experience by reducing execution time.

Testing

• Tested locally
• Added/updated tests
• Added/updated docs

[noah-thor]

Local execution time saw a decrease of ~60s

hyperfine \
                                                                               --min-runs 3 \
                                                                               --prepare 'make clean; make all' \
                                                                               --export-json integ-bench.json \
                                                                               --command-name 'per-suite (make integration)' \
                                                                                 'make integration' \
                                                                               --command-name 'single-process (make integration-fast)' \
                                                                                 'make integration-fast'
Benchmark 1: per-suite (make integration)
  Time (mean ± σ):     636.377 s ± 28.689 s    [User: 134.902 s, System: 76.158 s]
  Range (min … max):   617.463 s … 669.387 s    3 runs

Benchmark 2: single-process (make integration-fast)
  Time (mean ± σ):     582.090 s ± 24.498 s    [User: 115.609 s, System: 64.499 s]
  Range (min … max):   553.825 s … 597.187 s    3 runs