tests/micropython/opt_level: Add test for opt_level 3.

tests/micropython/opt_level.py

@@ -12,3 +12,7 @@ exec('print(__debug__)')
 micropython.opt_level(1)
 exec('print(__debug__)')
 exec('assert 0')
+
+# check that level 3 doesn't store line numbers
+micropython.opt_level(3)
+exec('try:\n xyz\nexcept NameError as er:\n import sys\n sys.print_exception(er)')

tests/micropython/opt_level.py.exp

@@ -2,3 +2,6 @@
 1
 True
 False
+Traceback (most recent call last):
+  File "<string>", line 1, in <module>
+NameError: name 'xyz' is not defined