Permit use a single file to write faborication.

I prefer to write all faborications into one single file.

Current we only have options which specify a folder to load faborications.

so, for now, my solution is, create a file, test/fabricators/all_fabricator.rb, and add all faborications into it.

But, i hope we can default support a file, e.g. test/faborications.rb, which can be search and load if exists

as done by FactoryBot, which support test/factories.rb.

Thank you.