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/shells/bash/push.yml

@@ -4,6 +4,9 @@ env:
 jobs:
   check:
     runs-on: ubuntu-latest
+    defaults:
+      run:
+        shell: ${{ env.MY_SHELL }}
     steps:
       - shell: ${{ env.MY_SHELL }}
         run: |
@@ -12,6 +15,12 @@ jobs:
           else
             exit 1
           fi
+      - run: |
+          if [[ -n "$BASH" ]]; then
+            echo "I'm $BASH!"
+          else
+            exit 1
+          fi
   check-container:
     runs-on: ubuntu-latest
     container: node:24-bookworm-slim
@@ -23,15 +32,3 @@ jobs:
           else
             exit 1
           fi
-  check-job-default:
-    runs-on: ubuntu-latest
-    defaults:
-      run:
-        shell: ${{ env.MY_SHELL }}
-    steps:
-      - run: |
-          if [[ -n "$BASH" ]]; then
-            echo "I'm $BASH!"
-          else
-            exit 1
-          fi