Add support for running dotnet test just before dotnet publish

[dpozimski]

Would be awesome if developer can configure build pack to run unit tests (by specifying a pattern for unit tests project search) just before app publish to the heroku.
Currently It can be done via external CI step.

[jincod]

Hello @dpozimski,

I think the best place for dotnet test is a step at Heroku CI.

We already have a PR #77 but it hasn't been tested yet.