add note to tests

Author: BobTheBuidler

mypyc/test-data/irbuild-classes.test

@@ -2690,12 +2690,15 @@ L0:
 [case testInvalidMypycAttr]
 from mypy_extensions import mypyc_attr
 
-@mypyc_attr("allow_interpreted_subclasses", "invalid_arg")  # E: 'invalid_arg' is not a supported `mypyc_attr`
+@mypyc_attr("allow_interpreted_subclasses", "invalid_arg")  # E: 'invalid_arg' is not a supported `mypyc_attr` \
+                                                            # N: supported keys: 'allow_interpreted_subclasses', 'free_list_len', 'native_class', 'serializable'
 class InvalidArg:
     pass
-@mypyc_attr(invalid_kwarg=True)  # E: 'invalid_kwarg' is not a supported `mypyc_attr`
+@mypyc_attr(invalid_kwarg=True)  # E: 'invalid_kwarg' is not a supported `mypyc_attr` \
+                                 # N: supported keys: 'allow_interpreted_subclasses', 'free_list_len', 'native_class', 'serializable'
 class InvalidKwarg:
     pass
-@mypyc_attr(str())  # E: All `mypyc_attr` args must be string literals.
+@mypyc_attr(str())  # E: All `mypyc_attr` args must be string literals. \
+                    # N: supported keys: 'allow_interpreted_subclasses', 'free_list_len', 'native_class', 'serializable'
 class InvalidLiteral:
     pass