Added test for '--posix-captures': implicit grouping is forbidden.

Added test for '--posix-captures': implicit grouping is forbidden.

RE2C has named definitions, which allow to override operator precedence
without grouping parenthesis. However, in POSIX all parenthesis are
capturing: allowing some of them to be non-capturing would affect
disambiguation and might result in a different parse.

For now RE2C uses a crude criterion to determine if the given named
definition needs to be explicitely wrapped in parenthesis: if the topmost
constructor of the subexpression is either alternative, or catenation, or
another reference to another named definition, then wrapping is required.
This is an overapproximation, because it doesn't take the surrounding context
into account (e.g. catenation inside of an alternative doesn't need wrapping,
but catenation inside of an iteration does).