Snapp integration test

[psteckler]

Create an integration test for snapps.

Strategy:

• generate a Parties.t using QC generators
• integration test ledgers have a snapp field
• substitute fee payer key from an account in the integration test ledger
• substitute a dummy signature for the fee payer, so the snapp is expected to fail

Other changes:

• add a new GraphQL endpoint sendTestSnapp that accepts a scalar, so programatically, we can just send a Yojson.Basic.t; the extant sendSnapp requires an OCaml object ; the upcoming derivers may make this extra endpoint unnecessary
• the length check on token symbols updated to use nonstrict inequality
• define snapp_to_be_included_in_frontier for future tests
• disable profile_constraints in Pickles due to an exception opening a log file; added a close for that file, which didn't help

Part of #10067.