CMake: Update build tools python modules to pick refactored way of unittest CMake

- Update mbed_unittest python module to pick unittest from mbed os root
CMake configuration file
- Add BUILD_TESTING flag ON in test python module

UNITTESTS/mbed_unittest.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 """
-Copyright (c) 2018, Arm Limited
+Copyright (c) 2018-2021, Arm Limited
 SPDX-License-Identifier: Apache-2.0
 
 Licensed under the Apache License, Version 2.0 (the "License");
@@ -71,9 +71,8 @@ def _mbed_unittest_test(options, cwd, pwd):
                                  clean=options.clean)
 
     if options.compile_only:
-        # Create makefiles
-        src_path = os.path.relpath(pwd, options.build)
-        tool.create_makefiles(path_to_src=src_path,
+        # Create makefiles        
+        tool.create_makefiles(path_to_src=cwd,
                               generator=options.cmake_generator,
                               coverage_output_type=options.coverage,
                               debug=options.debug_build,

UNITTESTS/unit_test/test.py

@@ -86,6 +86,7 @@ class UnitTestTool(object):
         args = [cmake,
                 "-G",
                 generator,
+                "-DBUILD_TESTING=ON"
                 "-DCMAKE_MAKE_PROGRAM=%s" % self.make_program,
                 "-DCMAKE_CXX_COMPILER=%s" % get_cxx_tool(),
                 "-DCMAKE_C_COMPILER=%s" % get_c_tool()]