Add tests for printing of NonZero-optimized enums in Rust

gdb/testsuite/ChangeLog:
2016-06-25  Manish Goregaokar  <manish@mozilla.com>

    PR gdb/20239
    * gdb.rust/simple.rs: Add more tests for printing NonZero enums.
    * gdb.rust/simple.exp: Add test expectations for new NonZero tests.