Commit 0841477 moved ServerEndpoint to an independent package. Move its
tests over as well.
Updates tailscale/corp#27502
Signed-off-by: Jordan Whited <jordan@tailscale.com>
ServerEndpoint will be used within magicsock and potentially elsewhere,
which should be possible without needing to import the server
implementation itself.
Updates tailscale/corp#27502
Signed-off-by: Jordan Whited <jordan@tailscale.com>