CMake: connectivity: Add drivers subdirectory for tests

Greentea tests run on microcontrollers, so any required drivers need
to be available and built.

Note: Even though unit tests do not require drivers, for ease of
maintenance there's no need to exclude those drivers when building unit
tests because a driver only gets enabled when the Mbed target matches.

connectivity/CMakeLists.txt

@@ -27,6 +27,7 @@ add_library(mbed-wifi INTERFACE)
 if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
     # Add these subdirectories for tests
     add_subdirectory(cellular)
+    add_subdirectory(drivers)
     add_subdirectory(FEATURE_BLE)
     add_subdirectory(libraries)
     add_subdirectory(lorawan)