Update generate:stdlib_test

[pocke]

This pull request contains several fixes for generate:stdlib_test rake task.

First, it raises an error for accessing unknown constant.
Because it requires rbs library but it accesses to Ruby::Signature constant.
So I replaced Ruby::Signature with RBS.

Second, it generates the old-style test cases, which uses StdlibTest. So I replaced it with the new style test cases.

Third, it isn't aware of libraries. It means the generator does not work for the standard libraries. For example, rake generate:stdlib_test[Pathname] doesn't work.

So I updated the generator to load all standard libraries.
It also loads unnecessary libraries, but I think it is the easiest way.

[soutaro]

👏 👏

[soutaro]

I'd suggest adding a simple test for the task, like running rake generate:stdlib_test[TESTTESTTEST] and see if it finished without error. @pocke

[pocke]

I'd suggest adding a simple test for the task, like running rake generate:stdlib_test[TESTTESTTEST] and see if it finished without error. @pocke

Thanks for your suggestion! It is a good idea 👍

I've added a simple test in 4072499.

[soutaro]

😄