Re-enable a ppx version test

[georgeee]

PR #15616 is going to fix ppx version tests' execution. However test bad_version_syntax_missing_versioned was commented out because it fails.

Some more information in the comment.

After PR #15616 is merged, we need to decide which way to change ppx version: either make it enforce an additional invariant which test was written for, or ditch the test and call it a day.