Merge every provider's unit tests project into the same library

[asiffermann]

There is no need to isolate unit tests in too much projects, we should merge them by library:

Proffer.Email.Tests

• InMemory
• SendGrid
• Smtp

Proffer.Storage.Tests

• Azure.Blobs
• FileSystem.Properties.Json
• FileSystem.Server
• FileSystem

Proffer.Templating.Tests

• Handlebars
• Mustache