Currently, tests of the same kind are grouped together across all types
into a single function. This makes it difficult to understand exactly
what failed in CI.
Change test macros to create separate functions for separate types so
failures are more fine grained.
Change float test macros to fall back to testing against `rustc_apfloat`
when system implementations are not available, rather than just skipping
tests.
This allows for easier debugging where operations may not be supported.
