Update to run "coretests"

[RalfJung]

rust-lang/rust#135937 moved the libcore tests to a separate crate, which probably means the test suite in this repo now just runs the libcore doc tests but not the unit tests.

@bjorn3 any chance you could have a look at this?

[bjorn3]

With rust-lang/rust#137679 ./x.py test library/core will work again.

[RalfJung]

That's cool but doesn't help for this issue. :) This repo directly invokes the tests, it does not go through x.py.