AutoUpdate: tokens and tests

[HebaruSan]

Problem

#2344 fixed a JSON parsing problem that broke the auto updater for v1.24.0. This problem could and should have been caught by existing tests, but they weren't running on Travis, and when they did run, they failed due to TLS errors in their network requests (also fixed in #2344) rather than the real problem affecting the client.

#2263 added support for GitHub API auth tokens, but they are not currently used by the auto updater.

Cause

AutoUpdate has tests, but they're in the FlakyNetwork category and therefore don't run on Travis. This makes it harder to notice that you've broken the auto updater. When these tests do run, they rely on the network to provide input data, so failures cause by network problems can mask problems with other parts of the logic such as the JSON parsing.

AutoUpdate is not structured to be testable. It has many different responsibilities (checking for updates, parsing update metadata, downloading new client, running new client), which can't be separated out for isolated testing.

Changes

Now AutoUpdate is refactored to better isolate the network-dependent part of what it does, and a new test is added that exercises the parsing code without relying on the network at all. This test would have caught the problem that #2344 fixed, and now it will run on Travis every time.

One of the old tests is removed because it only fetched network data using functions that were removed in the refactoring (for a repo with no releases), but the rest are still there, so you can still use them to perform combined network-and-parsing tests if needed.

Net.DownloadText is updated to support auth tokens, and AutoUpdate uses it to get release info. So the auto updater now supports auth tokens.