Increase test memory allowance check

[FreddieAkeroyd]

Increase acceptable memory usage - test failed recently with

AssertionError: Expected commit size to be less than values: {'block_server.py': 900000, 'database_server.py': 900000}. Actually got: {'database_server.py': 910766.08, 'block_server.py': 918765.568}

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