Move tests to a separate project in blobstorage-backupdata

[JamieMagee]

The BackupData project currently has unit tests mixed in with production code. BackupJobTests.cs sits alongside BackupJob.cs, and the .csproj includes test dependencies (NUnit, Moq, FluentAssertions) that get bundled into the production build.

This causes a few problems:

• The deployed container includes test assemblies and dependencies it doesn't need
• Test code can accidentally reference internal implementation details without realizing it
• Running dotnet test and dotnet run operate on the same project, which gets confusing

Proposed structure:

tools/blobstorage-backupdata/
├── src/
│   └── BackupData/
│       ├── BackupData.csproj
│       ├── Program.cs
│       ├── BackupJob.cs
│       ├── Log.cs
│       └── JObjectExtensions.cs
├── tests/
│   └── BackupData.Tests/
│       ├── BackupData.Tests.csproj
│       ├── BackupJobTests.cs
│       └── (test helpers)
└── BackupData.sln

The test project would reference the main project and contain all the NUnit/Moq/FluentAssertions dependencies.