- removed implicit tests based on SubdomainLocalhostGatewayURL
- subdomain tests are now run only against origins defined via
  `--subdomain-url`
- DNSLink gateway tests are no longer tied to origin passed via
  `--subdomain-url`

- splitting into subdomain and dnslink gateway variants
- towards avoiding running everything three times
- keep custom Host header when defined

these concepts should not mix, and dnslinks should be testable
independent of --subdomain-url

i also added more useful export format for them, compatible with
IPNS_NS_MAP used by Boxo/Kubo/Rainbow

towards fixing underlying issue

all-in-one command for running local kubo with all fixtures in

This replaces magic helpers.UnwrapSubdomainTests with explicit
spec preset named proxy-gateway, which can be enabled/disabled
the same way as other presets.

While it was nice to run every test in both proxy modes,
it was extremely noisy and hard to debug once things went wrong.

By having explicit tests in *_proxy_test.go it is easy to skip them
where proxy functionality is not relevant.

Other:

- Send meaningful User-Agent
- Sent detached path in HTTP Connect proxy tunnel tests (instead of full
  URL – in tunnel mode we pretend we talk to remote server on the other
  end of tunnel, and not send full URL like it is done in plain proxy
  mode)

This is important UX change. We no longer ship with default URLs.

User has to provide explicit one, or the test suite will refuse to run.
This ensures misconfigurations and testing different gateway endpoint
than desired do not happen.

Explicit is better than implicit.