tstest/tailmac: add support for mounting host directories in the guest (#13957)

updates tailscale/corp#24197

tailmac run now supports the --share option which will allow you
to specify a directory on the host which can be mounted in the guest
using  mount_virtiofs vmshare <path>.

Signed-off-by: Jonathan Nobels <jonathan@tailscale.com>

tstest/tailmac/Swift/Host/HostCli.swift

@@ -19,10 +19,12 @@ var config: Config = Config()
 extension HostCli {
     struct Run: ParsableCommand {
         @Option var id: String
+        @Option var share: String?
 
         mutating func run() {
-            print("Running vm with identifier \(id)")
             config = Config(id)
+            config.sharedDir = share
+            print("Running vm with identifier \(id) and sharedDir \(share ?? "<none>")")
             _ = NSApplicationMain(CommandLine.argc, CommandLine.unsafeArgv)
         }
     }