test(e2e): cover host mode, reusable workflows, scheduling, reconnect and CLI flags (#1347)

Extends the e2e suite with scenarios for host labels, local `workflow_call`, label routing, `runner.capacity`, a dropped and restored connection to Gitea, and the daemon's `--once`, `--labels` and `shutdown_timeout` behaviour.

Stacked on https://gitea.com/gitea/runner/pulls/1346, so its commit shows here until that merges. The new scenarios pass locally against gitea/gitea:latest and main-nightly.

Assisted-by: Codet:codet
Co-authored-by: Zettat123 <[email protected]>
Co-authored-by: bircni <[email protected]>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1347
Reviewed-by: Zettat123 <[email protected]>

e2e/testdata/workflows/capacity.yml

@@ -0,0 +1,17 @@
+name: capacity
+on: push
+jobs:
+  one:
+    runs-on: e2e-capacity
+    steps:
+      - run: |
+          echo "span-start=$(date +%s%N)"
+          sleep 8
+          echo "span-end=$(date +%s%N)"
+  two:
+    runs-on: e2e-capacity
+    steps:
+      - run: |
+          echo "span-start=$(date +%s%N)"
+          sleep 8
+          echo "span-end=$(date +%s%N)"