mp_compile no longer takes an emit_opt argument, rather this setting is now
provided by the global default_emit_opt variable.
Now, when -X emit=native is passed as a command-line option, the emitter
will be set for all compiled modules (included imports), not just the
top-level script.
In the future there could be a way to also set this variable from a script.
Fixes issue #4267.
Tinytest is classical assert-style framework, but MicroPython tests work
in different way - they produce content, and that content should be matched
against expected one to see if test passes. upytesthelper exactly adds
helper functions to make that possible.
Damien George