When running under TeamCity use its NUnit task so that test stats are collected

Initially I added NUnit build step in TeamCity but then saw that build.bat etc. already ran unit tests.

After discussing with Jake, instead adapt MSBuild file to report unit test results to TeamCity.

This is the second pull request for this issue, I closed the previous one because it used an old version of NUnit on TeamCity - the version had to be specified explicitly.