tstest/tailmac: add headless mode for automated VM testing

Add a --headless flag to the Host.app Run subcommand for running
macOS VMs without a GUI, enabling use from test frameworks.

Key changes:

  - HostCli.swift: When --headless is set, run the VM via VMController
    + RunLoop.main.run() instead of NSApplicationMain. Using the
    RunLoop (not dispatchMain) is required because VZ framework
    callbacks depend on RunLoop sources.

  - VMController.swift: Add headless parameter to createVirtualMachine
    that configures a single socket-based NIC (no NAT NIC). This
    matches the NIC configuration used when creating/saving VMs, so
    saved state restoration works correctly. A NIC count mismatch
    causes VZ to silently fail to execute guest code.

  - TailMacConfigHelper.swift: Clean up socket network device logging.

  - Config.swift: Move VM storage from ~/VM.bundle to
    ~/.cache/tailscale/vmtest/macos/.

  - TailMac.swift: Fix dispatchMain→RunLoop.main.run() in the create
    command (same VZ RunLoop requirement).

Updates #13038

Change-Id: Iea51c043aa92e8fc6257139b9f0e2e7677072fa2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/tailmac/Swift/TailMac/TailMac.swift

@@ -329,7 +329,7 @@ extension Tailmac {
                 }
             }
 
-            dispatchMain()
+            RunLoop.main.run()
         }
     }
 }