tests/micropython/opt_level: Clarify the expected output for opt_level == 3.

tests/micropython/opt_level.py

@@ -14,5 +14,6 @@ exec('print(__debug__)')
 exec('assert 0')
 
 # check that level 3 doesn't store line numbers
+# the expected output is that any line is printed as "line 1"
 micropython.opt_level(3)
 exec('try:\n xyz\nexcept NameError as er:\n import sys\n sys.print_exception(er)')