ipv4 tests are run twice due to module inclusion in ipv6 test file

[bruingineer]

cargo test ipv4 ... runs the ipv4 tests twice. Once through the main ipv4 file and then again because of the pub mod ipv4_tests inside the ipv6 test file. The import is only used to access constants. It may be worth to have duplicate constants if no other test de-duplication solution exists.

[bruingineer]

addressed by updating the run tests command to be cargo test --test ipv4_tests -- ... so the only the tests contained in ipv4_tests.rs are run. fixed in #41