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/cancellation_test.go

@@ -107,17 +107,14 @@ func TestMainStepsExecutorMarksFailedOnTimeoutBetweenSteps(t *testing.T) {
 		"job1": createJob(t, `runs-on: ubuntu-latest`, ""),
 	})
 
-	// A short deadline that we let elapse between steps, so no step records the error itself.
-	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
-	defer cancel()
+	ctx := newControllableDeadlineContext(context.Background())
 
 	var ran []string
 	var laterStepCtxErr error
 	steps := []common.Executor{
-		func(c context.Context) error {
+		func(context.Context) error {
 			ran = append(ran, "step1")
-			// Block until the job deadline elapses, then return cleanly: the interrupt lands in the loop's between-steps check, not inside a step.
-			<-c.Done()
+			ctx.expire()
 			return nil
 		},
 		func(c context.Context) error {