[Coverage] BuildBot.Discord — increase code coverage to 100%

[credfeto]

Problem

BuildBot.Discord has 0% line coverage and 0% branch coverage. There are no tests for any of the types in this assembly.

Closes part of #356.

Current State

• Line coverage: 0%
• Branch coverage: 0%

Files Requiring Tests

All source files in src/BuildBot.Discord/:

• BotMessage.cs
• BotReleaseMessage.cs
• BotService.cs
• DiscordBot.cs
• DiscordBotConfiguration.cs
• DiscordBotLoggingExtensions.cs
• DiscordBotMessageNotificationHandler.cs
• DiscordBotMessageNotificationHandlerLoggingExtensions.cs
• DiscordBotReleaseMessageNotificationHandler.cs
• DiscordSetup.cs
• IDiscordBot.cs
• IMessageChannel.cs
• MessageChannel.cs

Acceptance Criteria

• A new BuildBot.Discord.Tests project exists.
• All source types in BuildBot.Discord are covered at 100% line and branch coverage.
• All tests pass in CI.