Add tests to improve coverage of existing functionality

[3esmit]

Tests to be written:

• [ ] tests for process epoch:
  • [x] should update pending rewards
  • [x] should update total supply
  • [ ] should update total supply multiplier points
  • [x] should increase epoch
• [ ] tests for process account:
  • [x] should update user multiplier points
  • [ ] should update user epoch
  • [ ] should pay user proportional rewards
• [ ] tests for stake
  • [ ] should update user lockup time
  • [ ] should mint bonus multiplier points
  • [ ] should update bonus multiplier points (restake)
  • [ ] should update total supply multiplier points
  • [ ] should update total supply balance
  • [ ] should update total supply
• [ ] tests for lock
  • [ ] should update user lockup time
  • [ ] should update bonus multiplier points
  • [ ] should update total supply multiplier points
• [ ] tests for unstake
  • [ ] should burn multiplier points
  • [ ] should update total supply multiplier points
  • [ ] should update total supply balance
  • [ ] should update total supply

[0x-r4bbit]

@3esmit FYI: I've updated the issue description so it's a bit more actionable.