fix(tests): Small fixes for BasisTradingModule audit changes

[cgewecke]

PR points at #232

@0xSachinK Here are couple loose ends as discussed in Slack

• Add fix for _updateExternalPositionUnit
• Add test for negative balance check in _calculateNetFeesPositionUnit

[0xSachinK]

LGTM! Thanks for this @cgewecke. 👏🏻