tests/basics/builtin_help: Add test for help('modules').

tests/basics/builtin_help.py

@@ -13,5 +13,6 @@ help(int) # help for a class
 help(1) # help for an instance
 import micropython
 help(micropython) # help for a module
+help('modules') # list available modules
 
 print('done') # so last bit of output is predictable