tests: Add a suite of tests specifically for the pyboard.

In tests/pyb is now a suite of tests that tests the pyb module on the
pyboard.  They include expected output files because we can't run
CPython on the pyboard to compare against.

run-tests script has now been updated to allow pyboard tests to be run.
Just pass the option --pyboard.  This runs all basic, float and pyb
tests.  Note that float/math-fun.py currently fails because not all math
functions are implemented in stmhal/.