gccrs: Add necessary hooks for a Rust front-end testsuite

This copies over code from other front-end testsuites to enable testing
for the rust front-end specifically.

	gcc/testsuite/
	* lib/rust-dg.exp: New.
	* lib/rust.exp: New.

Co-authored-by: Marc Poulhiès <dkm@kataplop.net>
Co-authored-by: Thomas Schwinge <thomas@codesourcery.com>