cmake: Set MBED_TEST_MODE privately

Set MBED_TEST_MODE as PRIVATE. We don't need MBED_TEST_MODE set for
applications, only tests built with the "mbed-os" project. This also
helps avoid the following error when building an application, given that
applications currently are built as part of the "mbed-os" project, due
to limitations with Mbed OS's toolchain support (see #14518 for more
information on toolchain support).

    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
+            PRIVATE
                 MBED_TEST_MODE
         )
     endif()