Failing unit tests might not print the output

FYI in case someone hits this.

After merging https://github.com/dotnet/iot/pull/789 build.cmd/build.sh will run unit tests as part of the build but unfortunately due to bug in VSTest the output most of the time will not get printed (depends which worker picks up the job).

This is causing that if any of the unit test fails the output might not get printed and it's hard to diagnose what went wrong. Most of the time just passing `-v d` will fix the issue. Using `dotnet test` directly on the project always prints the output correctly.

VSTest issue: https://github.com/microsoft/vstest/issues/2224