CMake: Unit tests are always built if CMAKE_CROSSCOMPILING is OFF

[rwalton-arm]

Description of defect

Typically when adding a unit test directory to a CMake project a check
will be used to ensure the subdirectory is added only if the following
are true:

• The BUILD_TESTING option is set to ON.
• The current CMake project is the top-level project.

The reason being, if a downstream project includes our project they
generally don't want to build our unit tests.

In mbed-os, we do correctly specify the above condition before adding
the central UNITTEST subdirectory, which fetches googletest and adds the
"stub" libraries the unit tests depend on. However, we only check if
CMAKE_CROSSCOMPILING is OFF (or undefined) before actually adding the
unit tests. This mismatched logic would lead to unexpected build
failures in various scenarios. One likely case could be: a downstream
project including mbed-os happens to set CMAKE_CROSSCOMPILING to
OFF/undefined for any reason (possibly to build its own unit tests).
mbed-os would go ahead and attempt to build its tests without fetching
googletest or adding the required stub targets.

Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

All

What version of Mbed-os are you using (tag or sha) ?

master

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

Disable CMAKE_CROSSCOMPILING and run a build in an application including mbed-os.