Epoch manager enabler tests

[soloseng]

Description

Added basic unit test for epochManagerEnabler contract

Other changes

removed extra native token balance check in initializeSystem of epochManager contract.

Tested

unit tested

Related issues

• Fixes https://github.com/celo-org/celo-blockchain-planning/issues/593

Backwards compatibility

Brief explanation of why these changes are/are not backwards compatible.

Documentation

The set of community facing docs that have been added/modified because of this change

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