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)
Stupid typo
Changed the envvar name to MICROPY_MICROPYTHON

As discussed in #504
Merge pull request #504 from lurch/patch-4

Allow the uPy used by run-tests to be overridden