Add more tests

[HebaruSan]

Tests are added (catching regressions unless otherwise noted):

• Fallback to archive.org URLs for failed downloads of FOSS packages #2284 (archive.org URL formats)
• Replace colons with hyphens in archive URLs #2290
• Don't crash if download_hash isn't set #2313
• Fix ArgumentOutOfRangeException when removing files from game root #2332
• Obey version properties of conflicts and depends relationships in sanity checks #2339
• More verbose and accurate version displays #2382 / Loop to find max KSP version instead of assuming ordering #2131
• Don't double encode GitHub download URLs #2402
• Option to override SpaceDock version with AVC version #2406
• Game version range formats

The tests' names are of the form MethodBeingTested_Scenario_DesiredOutcome.
The code follows the Arrange / Act / Assert structure.

The tests for CKAN.CkanModule are moved from Module to CkanModuleTests so programmers familiar with CkanModule will have an easier time finding them.

Also courtesy of @DasSkelett (HebaruSan#4), references are added to Tests.csproj to allow running the tests directly in Visual Studio.

[politas]

Oooh! I like tests!

[DasSkelett]

Hey, may I open a pull request to your branch to add NUnit 3 Test Adapter to Tests/packages.config?
It's necessary to run NUnit tests inside VisualStudio.

[HebaruSan]

@DasSkelett, sure, bring it on!