Currently we have `MBED_TEST_LINK_LIBRARIES` for specifying
* Whether to link `mbed-os` or `mbed-baremetal`
* Any additional libraries we want tests to link

It's not fit for purpose anymore, because
* No flavor of Mbed OS is selected by default, but we should've
really defaulted to `mbed-os`, the full RTOS version. Build doesn't
work unless `-DMBED_TEST_LINK_LIBRARIES=<...>` is passed, which
is redundant.
* A test should never need additional libraries passed via command
line - its `CMakeLists.txt` should specify what it links.

This commit replaces `MBED_TEST_LINK_LIBRARIES` with a new option
`MBED_TEST_BAREMETAL` to build a test with either RTOS (default)
or without it (by passing `-DMBED_TEST_BAREMETAL=ON`).