Test core functions without importing EV3 module

Importing the EV3 module attempts to register LEDs, which fails in the test environment.

Author: WasabiFan

tests/api_tests.py

@@ -9,7 +9,7 @@
 from populate_arena import populate_arena
 from clean_arena    import clean_arena
 
-import ev3dev.ev3 as ev3
+import ev3dev.core as ev3
 
 ev3.Device.DEVICE_ROOT_PATH = os.path.join(FAKE_SYS, 'arena')