Fix tests.py so it can work with application defined targets

With this patch in place, tests.py uses targets names instead of target
instances, which makes it possible to use application defined targets
with tests.

tools/test.py

@@ -132,7 +132,7 @@ if __name__ == '__main__':
                 base_source_paths = ['.']
             
             
-            target = TARGET_MAP[options.mcu]
+            target = options.mcu
             
             build_report = {}
             build_properties = {}