test(e2e): speed up and cover exec and Kubernetes modes (#1363)

Follow-up to https://gitea.com/gitea/runner/pulls/1362, whose crash e2e missed because it only drove the daemon on Docker.

- `dispatch`, `failure_paths` and `reusable_workflow` run through the daemon and `exec`, each on Docker and one shared k3s cluster.
- Fixed sleeps become gates the test releases, the job timeout uses a 15s runner deadline, k3s boots alongside Gitea, Gitea's data is on tmpfs.
- Outside a container, `make test-e2e` runs `go test` in a Linux container on its own network like CI, so local runs take the same paths.

Local wall time:

| | Before | After |
|---|---|---|
| Suite | 84s | 27–35s |
| `job_timeout` | 64s | 17s |
| `reconnect` | 22s | 4.5s |
| `capacity/serial` | 20s | 2s |
| `cli_shutdown_drains` | 14s | 2.3s |
| `kubernetes` | 31s | 21–23s |

_Written by Claude (Opus 5.5)._

---------

Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1363
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

e2e/dispatch_test.go

@@ -11,27 +11,11 @@ import (
 	"time"
 )
 
-func testWorkflowDispatch(t *testing.T) {
-	t.Parallel()
-	ctx := t.Context()
-
-	api, repo := newScenario(t)
-	pushWorkflow(t, api, repo, "dispatch.yml")
-
-	branch, err := api.DefaultBranch(ctx, repo)
-	if err != nil {
-		t.Fatalf("get default branch: %v", err)
+func testWorkflowDispatch(t *testing.T, m mode) {
+	result := runWorkflow(t, m, workflowSpec{files: []string{"dispatch.yml"}, inputs: map[string]string{"subject": "dispatch-input-value"}})
+	if result.conclusion != "success" {
+		t.Errorf("run concluded %q, want success", result.conclusion)
 	}
-	inputs := map[string]string{"subject": "dispatch-input-value"}
-	if err := waitForDispatch(ctx, api, repo, "dispatch.yml", branch, inputs); err != nil {
-		t.Fatalf("dispatch workflow: %v", err)
-	}
-
-	dispatched := waitForRun(t, api, repo)
-	if dispatched.Event != "workflow_dispatch" {
-		t.Fatalf("run event is %q, want workflow_dispatch", dispatched.Event)
-	}
-	requireSuccess(t, api, repo, dispatched.ID)
 }
 
 // Gitea indexes workflows asynchronously after the Contents API commit.