Stupid typo

tests/run-tests

@@ -13,7 +13,7 @@ if os.name == 'nt':
     MP_PY = os.getenv('MICROPY_MP_PY', '../windows/micropython.exe')
 else:
     CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3')
-    MP_PY = ('MICROPY_MP_PY', '../unix/micropython')
+    MP_PY = os.getenv('MICROPY_MP_PY', '../unix/micropython')
 
 # Set of tests that we shouldn't run under Travis CI
 skip_travis_tests = set(['basics/memoryerror.py'])