Check our functions against `musl-math-sys`. This is similar to the
existing musl tests that go through binary serialization, but works on
more platforms.
Create a new test that checks `for_each_fn` against `ALL_FUNCTIONS`,
i.e. the manually entered function list against the automatically
collected list. If any are missing (e.g. new symbol added), then this
will produce an error.
We will have more test features in the near future, and it would be nice
for them all to have a common `test-` prefix. Reverse the existing
feature so this is the case.
There isn't any reason for this feature to be exposed or part of the
build script. Move it to a separate crate.
We will also want more tests that require some support functions; this
will create a place for them.
