Include CTest if IoT SDK is the top level project

Previously we included MCU-Driver-HAL as the top level CMake project.
This was to work around the the hard-coded inclusion of app.cmake to
enable toolchains in the HAL.

Because the HAL was technically the top level CMake project, we couldn't
use the typical check to conditionally include CTest to enable testing.
Instead we checked if the IOTSDK_HTRUN_ARGUMENTS variable was an empty
string.

The HAL has been reworked to use toolchain files passed on the command
line. So we don't have to include it before the IOT SDK project is
declared. Now IoT SDK is the actually the top level project. This means
we can use the standard check to include CTest and enable testing.

Signed-off-by: Robert Walton <rob.walton@arm.com>