FIX: Cast exception when Running all tests

When trying to run all tests, got
  Unable to cast object of type 'ImplemenationCTestTestAdapter.CTestCase' to type 'Microsoft.VisualStudio.TestPlatform.ObjectModel.TestCase'

Indeed, CTestCase do not inherit from TestCast (which is sealed) so we cannot
use Cast to convert from CTestCase to TestCase.
Use conversion operation inside Select instead
Merge pull request #2 from bcachet/feature/FIX_CastExceptionInRunTests

Looks good - merged.  Do you need the new version compiled to a vsix / uploaded to vs gallery or did you already install it yourself?