float: Use `assert_biteq!` where possible

`assert_eq!` ignores the sign of zero, but for any tests involving zeros
we do care about this sign. Replace `assert_eq!` with `assert_biteq!`
everywhere possible for float tests to ensure we don't miss this.
`assert_biteq!` is also updated to check equality on non-NaNs, to catch
the unlikely case that bitwise equality works but our `==`
implementation is broken.

There is one notable output change: we were asserting that
`(-0.0).fract()` and `(-1.0).fract()` both return -0.0, but both
actually return +0.0.

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