test: speed up tests (#1181)

Parallelize isolated workflow tests, consolidate redundant fixtures, and replace fixed waits with deterministic synchronization. Keep the readable curl service probe and use `getent` for hostname resolution.

Measured on the same machine with `make test`:

1. Wall time: 170.50s to 136.12s, down 34.38s or 20.2%.
1. `act/runner`: 138.417s to 124.291s, down 14.126s or 10.2%.
1. `act/runner` coverage: unchanged at 85.2%.
1. `TestDockerExecAbort`: 2.514s to 0.012s package time.

Stability checks:

1. Cancellation and deadline tests: 100 race-enabled repetitions.
1. Host runner suite: 10 race-enabled repetitions.
1. Changed Docker fixtures: 3 consecutive repetitions.

Full race suite, Go and Windows lint, source checks, and security scan pass.

Assisted-by: Codex:GPT-5
Reviewed-on: https://gitea.com/gitea/runner/pulls/1181
Reviewed-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>

act/runner/testdata/services/push.yaml

@@ -10,14 +10,10 @@ jobs:
         ports:
           - 80
     steps:
-      - name: Echo the Postgres service ID / Network / Ports
-        run: |
-          echo "id: ${{ job.services.postgres.id }}"
-          echo "network: ${{ job.services.postgres.network }}"
-          echo "ports: ${{ job.services.postgres.ports }}"
       - name: The job context describes the started containers
         run: |
           test -n "${{ job.container.id }}"
           test -n "${{ job.services.postgres.id }}"
+          test -n "${{ job.services.postgres.ports }}"
           test -n "${{ job.services.postgres.ports['80'] }}"
           test "${{ job.services.postgres.network }}" = "${{ job.container.network }}"