tsnet: TestLoopbackSOCKS5 is flaky on windows

### What is the issue?

In CI, the following test can fail every now and then:

```
--- FAIL: TestLoopbackSOCKS5 (5.61s)
tsnet_test.go:107: DERP httpsrv listener: 127.0.0.1:50363
tsnet_test.go:119: testcontrol listening on http://127.0.0.1:50365/
tsnet_test.go:385: socks connect tcp 127.0.0.1:50372->100.64.0.1:8081: unknown error general SOCKS server failure
stuntest.go:63: STUN server shutdown
```

You can see an example in this Actions run: https://github.com/tailscale/tailscale/actions/runs/5071355523/jobs/9107648282

### Steps to reproduce

Run CI or test on a windows machine.

### Are there any recent changes that introduced the issue?

Not that I know of.

### OS

Windows

### OS version

Whatever Actions uses

### Tailscale version

Latest from main

### Other software

_No response_

### Bug report

_No response_