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

[credfeto]

Problem

BuildBot.Watchtower 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.Watchtower/:

• WatchTowerPublishMessage.cs
• WatchTowerPublishMessageNotificationHandler.cs
• WatchtowerSetup.cs

Acceptance Criteria

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