tstest: do not error if the there are fewer goroutines than at start

This fixes test failures like:
```
    resource.go:34: goroutine count: expected 3, got 2
```

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f