Fix test failure with Rust 1.18 and 1.19

Fix test failure with Rust 1.18 and 1.19

With Rust 1.18 and 1.19, I saw some test suite failures.  They were
all of the same form -- Box seems to be qualified in the output now,
like:

  print box_some
  $64 = core::option::Option<alloc::boxed::Box<u8>>::Some(0x7ffff6c21018 "\001\000")

... where the test was expecting Option<Box<u8>>.

This patch fixes the problem in a way that should work with earlier
versions of Rust.

gdb/testsuite/ChangeLog
2017-05-18  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Allow Box to be qualified.