Allow the uPy used by run-tests to be overridden

with MICROPY_MP_PY envvar, in an analogous way to MICROPY_CPYTHON3 envvar.

(the reason for this will be made clearer by a later PR)

tests/run-tests

@@ -10,10 +10,10 @@ from glob import glob
 # to the correct executable.
 if os.name == 'nt':
     CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3.exe')
-    MP_PY = '../windows/micropython.exe'
+    MP_PY = os.getenv('MICROPY_MP_PY', '../windows/micropython.exe')
 else:
     CPYTHON3 = os.getenv('MICROPY_CPYTHON3', 'python3')
-    MP_PY = '../unix/micropython'
+    MP_PY = ('MICROPY_MP_PY', '../unix/micropython')
 
 # Set of tests that we shouldn't run under Travis CI
 skip_travis_tests = set(['basics/memoryerror.py'])
@@ -39,7 +39,7 @@ if test_on_pyboard:
     pyb = pyboard.Pyboard('/dev/ttyACM0')
     pyb.enter_raw_repl()
 
-running_under_travis = os.environ.get('TRAVIS', 'false') == 'true'
+running_under_travis = os.getenv('TRAVIS') == 'true'
 
 for test_file in tests:
     if running_under_travis and test_file in skip_travis_tests: