Scancode found missing license notices in our source files. This commit
addresses those issues by adding an Apache-2.0 notice to source files
highlighted.
* Move mbed-client-randlib/ headers into include/
(Note: we don't rename it to "randlib" because this library
is mirrored to https://github.com/ARMmbed/mbed-client-randlib,
and "mbed-client-randlib" may be reference by some projects)
* Move the standalone local unit test into tests/unit