tests/basics/struct_micropython: Add test for 'S' typecode in ustruct.

The 'S' typecode is a uPy extension so it should be grouped with the other
extension (namely 'O' typecode).  Testing 'S' needs uctypes which is an
extmod module and not always available, so this test is made optional and
will only be run on ports that have (u)struct and uctypes.  Otherwise it
will be silently skipped.