tests: Mollusk Merge tests (6/9)

[rustopian]

Part 6 of https://github.com/orgs/solana-program/projects/8.

(cumulative diff)

Problem

solana-program-test is on its slow way out.

Solution

Convert these tests to Mollusk, including simple but perfectly equivalent stake tracking.

This part migrates the Merge test. Only the program_test.rs test (which is really just a smoke test) is migrated.

test_missing_signers functionality is tweaked to infer Check::success() only on None, not [].