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/gitea_api.go

@@ -53,12 +53,16 @@ func (a *GiteaAPI) CreateRepo(ctx context.Context, name string) error {
 }
 
 func (a *GiteaAPI) CreateFile(ctx context.Context, repo, path, content, message string) error {
-	body := map[string]any{
-		"content": base64.StdEncoding.EncodeToString([]byte(content)),
-		"message": message,
+	return a.CreateFiles(ctx, repo, map[string]string{path: content}, message)
+}
+
+func (a *GiteaAPI) CreateFiles(ctx context.Context, repo string, files map[string]string, message string) error {
+	var operations []map[string]string
+	for path, content := range files {
+		operations = append(operations, map[string]string{"operation": "create", "path": path, "content": base64.StdEncoding.EncodeToString([]byte(content))})
 	}
-	url := fmt.Sprintf("/api/v1/repos/%s/%s/contents/%s", giteaAdminUser, repo, path)
-	return a.doJSON(ctx, http.MethodPost, url, body, nil)
+	body := map[string]any{"files": operations, "message": message}
+	return a.doJSON(ctx, http.MethodPost, fmt.Sprintf("/api/v1/repos/%s/%s/contents", giteaAdminUser, repo), body, nil)
 }
 
 func (a *GiteaAPI) CreateTag(ctx context.Context, repo, name string) error {