Rollup merge of #141669 - tgross35:float-test-cleanup, r=RalfJung

float: Replace some approximate assertions with exact

As was mentioned at [1], we currently use `assert_approx_eq` for testing
some math functions that guarantee exact results. Replace approximate
assertions with exact ones for the following:

* `ceil`
* `floor`
* `fract`
* `from_bits`
* `mul_add`
* `round_ties_even`
* `round`
* `trunc`

This likely wasn't done in the past to avoid writing out exact decimals
that don't match the intuitive answer (e.g. 1.3 - 1.0 = 0.300...004),
but ensuring our results are accurate seems more important here.

[1]: rust-lang/rust#138087 (comment)

The first commit is a small bit of macro cleanup.

try-job: aarch64-gnu
try-job: x86_64-gnu-aux

Author: matthiaskrgr