Move tests to a separate project in blobstorage-backupdata

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.