Add test for require dynamic string

[aycabta]

The require dynamic string like below shouldn't be parsed:

prefix = 'path'
require "#{prefix}/a_library"

[hsbt]

I wonder why it is not parsed.

But it's existence behavior. I'm ok to merge this.

[aycabta]

I think the dyanmic string expansion with variable is heavy to parse for RDoc, because RDoc just parses Ruby code but doesn't execute. I guess what this is the reason of the behavior.

Memo: In the first RDoc commit of Ruby trunk at revision 5073, it uses eval for string in arguments. And at eb53cb6 of Git repository, it skips dynamic string in the same method.