CMake: greentea: Remove dependency on global MBED_PATH

The mbed_greentea_add_test macro required a greentea test to set an
MBED_PATH variable to the path of the mbed-os root directory, which it
attempts to add as a 'subdirectory' of the test project. We can instead
use CMake's built in CMAKE_CURRENT_LIST_DIR variable to deduce the path
to mbed-os relative to the current list file directory, removing the
need for greentea tests to set MBED_PATH.

tools/cmake/mbed_greentea.cmake

@@ -1,9 +1,9 @@
 # Copyright (c) 2020-2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
-
 set(MBED_CONFIG_PATH ${CMAKE_CURRENT_BINARY_DIR} CACHE INTERNAL "")
 
 include(${CMAKE_CURRENT_LIST_DIR}/app.cmake)
+set(MBED_ROOT ${CMAKE_CURRENT_LIST_DIR}/../.. CACHE INTERNAL "")
 
 # CMake Macro for generalizing CMake configuration across the greentea test suite with configurable parameters
 # Macro args:
@@ -34,8 +34,7 @@ macro(mbed_greentea_add_test)
         "${multipleValueArgs}"
         ${ARGN}
     )
-
-    add_subdirectory(${MBED_PATH} build)
+    add_subdirectory(${MBED_ROOT} build)
 
     add_executable(${MBED_GREENTEA_TEST_NAME})