Recaptcha test is Synapse-specific

The test for recaptcha currently cheats a little bit and relies on the fact that Synapse makes recaptcha verification requests even if captcha registration is disabled. This is convenient in the case of Sytest as it means we don't have to change Synapse's config and restart it halfway through testing, but not so great for other homeservers that actually disable this functionality when recaptcha is disabled in their config (i.e Dendrite).

https://github.com/matrix-org/sytest/blob/5d419de9d89abd95898b8190bd60d11fcb7eb79d/tests/11register.pl#L7-L70

The solution is to test this properly and to make it so that the config is changed and the homeserver is restarted before and after this test.

For now we're going to disable captcha registration in Dendrite and blacklist the test.

cc @babolivier

Start with tests/11register.pl, especially the recaptcha test described in the issue. Review how Sytest changes homeserver configuration and restarts homeservers, then make the test exercise disabled captcha registration without relying on Synapse's behavior. Done means the test works for homeservers such as Dendrite, with configuration restored afterward.

Written by the indexing model from the issue text.