tests/micropython: Add test for micropython.opt_level() function.

tests/micropython/opt_level.py

+import micropython as micropython
+
+# check we can get and set the level
+micropython.opt_level(0)
+print(micropython.opt_level())
+micropython.opt_level(1)
+print(micropython.opt_level())
+
+# check that the optimisation levels actually differ
+micropython.opt_level(0)
+exec('print(__debug__)')
+micropython.opt_level(1)
+exec('print(__debug__)')
+exec('assert 0')

tests/micropython/opt_level.py.exp

+0
+1
+True
+False