Test the functions in OID module in Mbed Crypto

### Description

We don't directly test the functions in the OID module in Mbed Crypto. They only get tested indirectly through X.509 and TLS. We should write unit tests for these functions.

Note that test coverage tools may not give accurate results for `oid.c` because it uses macros to define many functions.

Related: #78 — test the OID values.

### Issue request type

[ ] Question
[x] Enhancement
[ ] Bug

Start by locating oid.c and the existing X.509 and TLS tests that currently exercise the OID functions indirectly. Add direct unit tests for the OID module functions, keeping in mind that macro-defined functions may make coverage reports inaccurate; done means the functions have direct test coverage.

Written by the indexing model from the issue text.