Re-export ruby-sys test feature and pass it on test builds #95
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This accompanies steveklabnik/ruby-sys#31.
Basically a re-export of a
ruby-sysfeature I added to conditionally linklibrubywhen it's needed (for running Rust tests). Normallylibrubyisn't needed due to how the interpreter dynamically loads c-extensions (see steveklabnik/ruby-sys#25).