Remove the src/test.rs implicit test.

It's not unreasonable to have unittests in a separate submodule of the
crate (being called `test` or `tests`), and having them in their own
file can be very sensible. Thus, the `src/test.rs` implicit default is
likely to trip up some perfectly reasonable use-cases. There's already
the `tests/...` default, so repairing a codebase after this removal is
just moving `src/test.rs` to `tests/whatever_name_you_want.rs`.

Closes #187.