Long_tests/qcheck_rpc: prevent stack overflows when parsing schemas

Context

When parsing json schemas in qcheck_rpc.ml to generate random RPC inputs, we had two references that were ultimately circular: "tree_encoding" and "inode_tree". Parsing those references would cause infinite recusrions in parse_input and crash the test suite.

This MR adds two special cases to parse_input, similar to those which are already there for references to "bignum" and "micheline.alpha.michelson_v1.expression".

However, note that the two new generators are trivial: they always produce a Null json value. Those generators will need to be correctly implemented in a future commit.

Part of #3281 (closed)

Manually testing the MR

Checklist

• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR