cmake: Set MBED_TEST_MODE with INTERFACE

Set MBED_TEST_MODE with INTERFACE. Use INTERFACE because mbed-os is an
interface target, and we can only set INTERFACE properties on interface
targets. This helps avoid the following error when building an
application.

    CMake Error at mbed-os/CMakeLists.txt:96 (target_compile_definitions):
      target_compile_definitions may only set INTERFACE properties on INTERFACE
        targets

Author: Patater

CMakeLists.txt

@@ -94,7 +94,7 @@ if(${CMAKE_CROSSCOMPILING})
     # Add MBED_TEST_MODE for backward compatibility with Greentea tests written for use with Mbed CLI 1
     if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
         target_compile_definitions(${PROJECT_NAME}
-            PUBLIC
+            INTERFACE
                 MBED_TEST_MODE
         )
     endif()