[binutils-gdb] Add Rust test case for ".." struct initializer

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 50146e7022cedafe615bfc9884358d291a00689f ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 50146e7022cedafe615bfc9884358d291a00689f

Add Rust test case for ".." struct initializer

Building with --coverage pointed out that there was no Rust test for
initializing a structure using the ".." initializer.  This patch adds
such a test.

Regression tested on x86-64 Fedora 26.

2018-04-11  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.exp: Add test for ".." struct initializer.