This testsuite is heavily inspired from the LTO testsuite that uses a
pattern where each file is compiled to an object file and finally linked
together. Since Rust does not have headers/prototypes, we rely on the
ordering here so that all files numbered greater than zero get compiled to
object files first. This leaves the _0 file free to test the 'extern crate' and
'use' keywords to force testing of the compiler to read metadata from the
other 'crates'.