[pseudo] Sort nonterminals based on their reduction order.

Reductions need to be performed in a careful order in GLR parser, to
make sure we gather all alternatives before creating an ambigous forest
node.

This patch encodes the nonterminal order into the rule id, so that we
can efficiently to determinal ordering of reductions in GLR parser.

This patch also abstracts to a TestGrammar, which is shared among tests.

This is a part of the GLR parser, https://reviews.llvm.org/D121368,
https://reviews.llvm.org/D121150

Differential Revision: https://reviews.llvm.org/D122303