Add tests for package

[MattKotsenas]

In preparation for #46, add some test to validate the current package behavior. This doesn't test everything; future PRs should ensure the code they're changing is tested.

The test flow is as follows:

1. Include DotNet.ReproducibleBuilds's own .props and .targets files into the test project's output
2. Create a temp directory
3. Create a {project}.*.props and {project}.*.targets file in the obj/ folder so that the .NET SDK will import the props and targets at the correct time (this is basically to do a NuGet restore of the project without needing to create the package or deal with package resolution)
4. Create a temp project and assert the behavior

[baronfel]

This looks great, no notes.

I really like that library - might have to bug @jeffkl and see if we can get it added to feeds that the SDK uses so we can migrate off our home-grown stuff :D