Changing build directory for tests to .build/tests.

This change is necessary because it allows mbed-cli to properly place the
build artifacts for each test case. Before, there were left over build
artifacts from previous builds in the main build directory that caused
issues for the next builds.

Author: bridadan

mbed/mbed.py

@@ -1277,7 +1277,7 @@ def compile(toolchain=None, mcu=None, source=False, build=False, compile_library
     if compile_tests:
         # Compile tests
         if not build:
-            build = os.path.join(os.path.relpath(root_path, orig_path), '.build', target, tchain)
+            build = os.path.join(os.path.relpath(root_path, orig_path), '.build/tests', target, tchain)
 
         popen(['python', os.path.join(tools_dir, 'test.py')]
             + ['-t', tchain, '-m', target]