Rollup merge of #149969 - jyn514:ibt, r=jieyouxu

don't use no_main and no_core to test IBT

The previous test was quite fragile and depended on a bunch of internal features. Simplify it.

Split out of rust-lang/rust#149937.

cc ```@jieyouxu``` ```@Oneirical```

Author: matthiaskrgr