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/scheduling_test.go

@@ -75,10 +75,16 @@ func testCapacity(t *testing.T) {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 			api, repo := newScenario(t)
+			gate, release := newGate(t)
+			if err := api.CreateVariable(t.Context(), repo, "E2E_GATE", gate); err != nil {
+				t.Fatalf("create gate variable: %v", err)
+			}
 			startRunner(t, repo, "e2e-capacity", runnerOptions{capacity: tc.capacity})
 			pushWorkflow(t, api, repo, "capacity.yml")
 
 			wfRun := waitForRun(t, api, repo)
+			waitForRunningJobLog(t, api, repo, wfRun.ID, "span-start=", tc.capacity)
+			release()
 			requireSuccess(t, api, repo, wfRun.ID)
 
 			jobs, err := api.Jobs(t.Context(), repo, wfRun.ID)