expression tests: adjust starred expression for Python 3.9 (#1441) (#1477)

As discussed in #1441, Python 3.9's new parser will not parse
`(*starred)` even using `compile()` with the `PyCF_ONLY_AST`
flag (as `ast.parse()` does), it raises a `SyntaxError`. This
breaks the four tests that use this file with Python 3.9.
Upstream does not consider this to be a bug - see
https://bugs.python.org/issue40848#msg370643

 - so we must
adjust the expression. As suggested by @JelleZijlstra, this just
adds a comma, which makes the new parser happy with it (the old
parser is fine with this form also).

Signed-off-by: Adam Williamson <awilliam@redhat.com>