auto merge of #15245 : sfackler/rust/coretest, r=alexcrichton

Libcore's test infrastructure is complicated by the fact that many lang
items are defined in the crate. The current approach (realcore/realstd
imports) is hacky and hard to work with (tests inside of core::cmp
haven't been run for months!).

Moving tests to a separate crate does mean that they can only test the
public API of libcore, but I don't feel that that is too much of an
issue. The only tests that I had to get rid of were some checking the
various numeric formatters, but those are also exercised through normal
format! calls in other tests.