gccrs: Move a test into compile/macros/mbe

This test should go in the folder for "macros by example", not the
top level rust compile test folder.

gcc/testsuite/ChangeLog:

	* rust/compile/attr-macro.rs: Move to...
	* rust/compile/macros/mbe/macro_attr_expr.rs: ...here.

Signed-off-by: Owen Avery <powerboat9.gamer@gmail.com>