Label scope specs: Add negative specs (subexpressions)
Label scope specs: Add negative specs (subexpressions)

Semantic predicate and action specs which verified label scope didn't
exercise labels in subexpressions. This commit adds cases exercising
them, including few commented-out cases which reveal #396 (these will be
uncommented when the bug gets fixed).

Note that added specs exercise all relevant expression types. This is
needed because code that makes subexpression labels invisible to the
outside is separate for each expression type so one generic test
wouldn't generate enough coverage.

Part of a fix of #396.

pegjs/pegjs,Sciumo/pegjs,pegjs/pegjs,Sciumo/pegjs,Mingun/pegjs,Mingun/pegjs