Auto merge of #141857 - RalfJung:coretests-floats, r=<try>

coretests: move float tests from num to floats module and use a more flexible macro to generate them

This makes some progress on #141726 by moving the float tests in `num` to `floats` and using a newer, more flexible macro to generate them. We also newly run these tests on f16 and f128 in const, and at runtime in Miri and for hosts where that works well enough.

I didn't yet deduplicate any tests or port the existing `floats::f*` tests to the macro, that can happen in a future PR.

<!-- homu-ignore:start -->
<!--
If this PR is related to an unstable feature or an otherwise tracked effort,
please link to the relevant tracking issue here. If you don't know of a related
tracking issue or there are none, feel free to ignore this.

This PR will get automatically assigned to a reviewer. In case you would like
a specific user to review your work, you can assign it to them by using

    r? <reviewer name>
-->
<!-- homu-ignore:end -->

try-job: x86_64-gnu-aux

Author: rust-bors[bot]