Generate testing assemblies at compile time

[slozier]

There are a number of tests (see #1662) that generate assemblies during testing. These could probably be generated at compile time (similar to what's done in the DLR ClrAssembly project). This would improve test coverage on systems that don't have csc/vbc/ilasm (e.g. a posix system without Mono).

[slozier]

Another option would be to also generate proj files which would allow us to compile on the fly using dotnet.