Cmake: Add MBED_TEST_MODE macro

Fixes #14494

MBED_TEST_MODE was used in CLI1 to prevent multiple applications from
being built. Add the macro to provide backward compatibility.

CLI1 Reference: https://os.mbed.com/docs/mbed-os/v6.9/debug-test/greentea-for-testing-applications.html

Author: harrisonmutai-arm

CMakeLists.txt

@@ -72,6 +72,14 @@ target_compile_definitions(mbed-core
         ${MBED_CONFIG_DEFINITIONS}
 )
 
+# Add MBED_TEST_MODE for backward compatibility with Greentea testing in CLI1
+if(CMAKE_PROJECT_NAME STREQUAL PROJECT_NAME AND BUILD_TESTING)
+    target_compile_definitions(${PROJECT_NAME}
+        PUBLIC
+            MBED_TEST_MODE
+    )
+endif()
+
 # We need to generate a "response file" to pass to the C preprocessor when we preprocess the linker
 # script, because of path length limitations on Windows. We set the response file and bind the path
 # to a global property here. The MBED_TARGET being built queries this global property when it sets