Add test suite for csproj modifications

We need a suite of tests based on the modifications we make to the .csproj files, to prevent regressions.

This will require isolating and mocking some Unity APIs, such as application version, target framework and OS checks. We can then create a folder per test, with an existing .sln and one or more .csproj files, response files, etc. and run for multiple versions of Unity itself - write the modified .csproj files and check for differences with gold files.