tests/run-tests: Run CPYTHON3 with -S flag.
tests/run-tests: Run CPYTHON3 with -S flag.

-S is "don't imply 'import site' on initialization". This avoids sys.path
minipulation and possibility of importing any module outside CPython
standard library. This is required for clean-room testing, as CPython
installtion may have user modules named "uio", "utime", etc.

Change-Id: Iac56852ec69b8db8a1f3fb2109e86fe7f28953b4

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython