Unskip testTrailingCommaParsing test

[sobolevn]

The first test right now passes.

But, the second one is not valid Python:

>>> if x in 1, : pass
  File "<stdin>", line 1
    if x in 1, : pass
             ^
SyntaxError: invalid syntax

And:

>>> import ast
>>> ast.dump(ast.parse('if x in 1, : pass'))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/sobolev/.pyenv/versions/3.8.9/lib/python3.8/ast.py", line 47, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 1
    if x in 1, : pass
             ^
SyntaxError: invalid syntax

So, we must raise a correct syntax error for it.