* MM-23666: rewrite TestHTTPClient
The existing `TestHTTPClient` actually tries to reach google.com, making it brittle to network hiccups (as unlikely as that might be). Rewrite the test to rely on a local server instead.
I found the existing implementation of `TestHTTPClient` extremely hard to understand, so I've rewritten it more explicitly, expanded coverage somewhat. In doing so, I found myself somewhat surprised by the current behaviour. If `allowHost` returns `true`, we ignore `allowIP` altogether. If `allowHost` returns `false`, we check with `allowIP`. There's a lack of symmetry here that the current filter names don't make clear. Perhaps `allowHost` should be renamed `skipIPCheckForHost`?
Fixes: https://mattermost.atlassian.net/browse/MM-23666
* more compact assertions
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