Adjust FA1.2 tests

Description

Problem: we have a test suite checking that at zero transfer no changes are applied to the contract, but this restriction is too strict and the original FA1.2 interface does not require such thing. Recently this became a problem in one of our projects.

Solution: move this test suite to ManagedLedger tests, and in general ApprovableLedger tests leave a test on that zero transfer just does not modify participants' balances.

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