Check the parsed AST in the tests

[DCNick3]

First of all, thanks for your great project, it helps me very much with parsing microsoft function definitions in their headers.

In spite of the last point of #59 I have some concerns regarding the test suite: it does not seem to check the resultant AST or generated code correctness.

I think it might be worthwhile to add such checks to all the tests. As of now I am considering replacing all the ast.show() and print(GnuCGenerator().visit(ast)) in the tests with checks against known good values.

The problem I see is that ast.show() results are incomplete (they lack attributes in some cases, for example), so it might not be the best idea to use it as-is.

What do you think regarding this issue? Maybe there is a better way to approach this issue?

[inducer]

I'd be happy for any and all improvements to the tests. I think the round-trip tests I proposed in #59 are a reasonable thing to pursue, but I'm open to improvements. I don't currently use this package in any of my work, so I can't put too much work into it myself, but I'm happy to review contributions. Especially ones that are self-evidently correct, based on their tests. :)