Add more tests

[tpetricek]

There are now separate test projects, but most of them are still just exe files that you can run to test things by hand. We should turn those into unit tests...

The only one that has the right structure is CodeFormat.Tests, so this can be used as an inspiration how the structure should look.

I'd welcome any contributions here :-) feel free to turn other projects in tests folder into proper Library projects that reference NUnit, include FsUnit.fs and contain some actual tests!

For Markdown, it would be nice to compare against some existing tools (there are some sample outputs in Benchmarks folder) but we do not necessarily have to do exactly the same thing...

[dungpa]

What kinds of tests do you have in mind?

I suppose we can turn pairs of input and output files into tests but it's restrictive since output formats may change and output files should fulfill certain properties rather than be fixed into some values.

We really need tests for Markdown parser. If we add pretty printing for ASTs, we can test the following property on a vast number of inputs:

 parse >> print >> parse = parse