Move Rust union tests to new file

Move Rust union tests to new file

I wanted to run the gdb.rust tests against older versions of the Rust
compiler, to ensure that changes I am making don't break debugging
when using older compilers.

However, this did not work because simple.rs now uses unchecked
unions, which were only added in Rust 1.19.

This patch splits the union code into its own file, so that simple.exp
can continue to work.  I tested this with selected rust versions back
to 1.12.

gdb/testsuite/ChangeLog
2020-04-01  Tom Tromey  <tromey@adacore.com>

	* gdb.rust/union.rs: New file.
	* gdb.rust/union.exp: New file.
	* gdb.rust/simple.rs (Union, Union2): Move to union.rs.
	(main): Update.
	* gdb.rust/simple.exp: Move union tests to union.exp.