Unit tests: Move randLIB.h

The stub header randLIB.h overrides the header of the same name in
platform/randlib/ which is an external repository vendored into
the mbed-os codebase. As the repository is synchronized regularly,
it is better not to put overrides there, so we put the randLIB.h
stub into the regular platform doubles directory.

Author: LDong-Arm