Some build system improvements to reference tests

* Reference tests can now be run with `msbuild /t:RefTest`
* Reference tests are now built when building the main project on Windows
* New test files in `Source\ReferenceTests` are picked up automatically in `PowerShellReferenceTests.csproj` and no longer need to be added manually
* Fixes a build error in `PowerShellReferenceTests.csproj` that was introduced in #204
* Minor improvements to the main project file