Add functional tests for application host

Follow @davidfowl 's suggestion here: https://github.com/aspnet/KRuntime/pull/808

add functional tests for `k` command to prevent any further regression.
CreateTempDir() should return an object that's disposable so that it'll be cleaned up automagically:

```
using(var temp = TestUtils.CreateTempDir())
{
      var dir = temp.Directory;
}
```