Fix Rust test suite for 1.20 beta

Fix Rust test suite for 1.20 beta

I ran the gdb.rust tests against Rust 1.20 (beta) and saw a few
failures.  The failures all came because a particular item moved to a
different module.  Since the particular choice of module name isn't
important here, I simply widened the allowable results.

Tested locally against rustc 1.19, 1.20, and 1.21.

testsuite/ChangeLog
2017-08-05  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Allow String to appear in a different
	namespace.