TestNewConn now passes as root on Linux. It wasn't closing the BPF
listeners and their goroutines.
The code is still a mess of two Close overlapping code paths, but that
can be refactored later. For now, make the two close paths more similar.
Updates #9945
Change-Id: I8a3cf5fb04d22ba29094243b8e645de293d9ed85
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>