Style: Remove drivers unit test directory from codeignore
Style: Remove drivers unit test directory from codeignore

Unit tests for drivers are currently not checked for style compliance.
We should be checking coding style in unit tests, so we remove them from
the .codecheckignore file.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed