This change adds setup for a second tailnet to enable multi-tailnet e2e
tests. When running against devcontrol, a second tailnet is created via the
API. Otherwise, credentials are read from SECOND_TS_API_CLIENT_SECRET.
Also adds an l7 HA Ingress test for multi-tailnet.
Fixestailscale/corp#37498
Signed-off-by: Becky Pauley <becky@tailscale.com>