Removed the static TestRunner constructor

OnClient is now a dynamic property and the logic in the static
constructor has been split between the Log method and the dynamic
constructor.
Also added a try-catch block for catching exceptions when TestRunner is
called from unit tests, i.e. outside a Starcounter application
Rewrote BuildVariableTest since TestRunner no longer has a static OnClient property

Also changed the environment variable scope to process instead of user