chore: make noopManager a no-op, even if a DNS config is supplied

[ethanndickson]

Re: coder/coder#14718

The Tailscale DNS manager that's used when you don't supply one to the wireguard engine (noopManager) returns false for whether it supports split DNS. If split DNS isn't supported, and a non-empty DNS config is supplied, the wireguard engine will try and get the base DNS config, which the no-op manager also doesn't support, and then logs an error, which fails a heap of coder/coder tests.

To address this, we can modify the default configurator to perform a no-op, even if a dns config is supplied.

We could also just set the DNS config conditionally (i.e. only for CoderVPN, or if a host has been supplied to the tailnet.Conn), but doing the plumbing for that seems not-great compared to just making this a no-op.

[ethanndickson]

• chore: make noopManager a no-op, even if a DNS config is supplied #63 👈
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[spikecurtis]

I do think its a good thing that we log errors if we try to program DNS entries into a tailnet.Conn that doesn't support it.

I'd rather set the DNS conditionally somehow. Maybe we look at the Options.DNSConfigurator to decide whether to set DNS config, and throw errors if you try to insert DNS records into a tailnet.Conn that doesn't have that option set?

[ethanndickson]

Yep, that works 👍