Remove redundant tests::tests mod

[Velnbur]

Problem

As file tests.rs already creates the module named tests there is no reason to have an additional one inside called the same (tests) as it creates for tests path like tests::tests.

Solution

Remove inner tests module and move #[cfg(test)] to declaration of mod tests

[olegfomenko]

Applied in 70382f6
Thanks @Velnbur