fix(button-token): fixing button-token dust leak and hardhat-coverage

[SocksNFlops]

Changes:

• Fixing dust-leak in button-tokens
• Fixing hardhat-coverage by upgrade solidity-coverage plugin

-----------------------------|----------|----------|----------|----------|----------------|
File                         |  % Stmts | % Branch |  % Funcs |  % Lines |Uncovered Lines |
-----------------------------|----------|----------|----------|----------|----------------|
 contracts/                  |    90.87 |    69.09 |    91.57 |    91.27 |                |
  ButtonToken.sol            |     84.5 |       70 |    88.89 |    85.81 |... 405,406,407 |
  ButtonTokenFactory.sol     |      100 |      100 |      100 |      100 |                |
  ButtonTokenWamplRouter.sol |      100 |       70 |      100 |      100 |                |
  ButtonTokenWethRouter.sol  |      100 |    64.29 |      100 |      100 |                |
  UnbuttonToken.sol          |    96.55 |    66.67 |    90.91 |    96.61 |        184,189 |
  UnbuttonTokenFactory.sol   |      100 |      100 |      100 |      100 |                |
 contracts/external/         |    27.78 |        0 |     37.5 |    29.09 |                |
  WAMPL.sol                  |    27.78 |        0 |     37.5 |    29.09 |... 211,217,223 |
 contracts/interfaces/       |      100 |      100 |      100 |      100 |                |
  IButtonToken.sol           |      100 |      100 |      100 |      100 |                |
  IButtonWrapper.sol         |      100 |      100 |      100 |      100 |                |
  IChainlinkAggregator.sol   |      100 |      100 |      100 |      100 |                |
  IFactory.sol               |      100 |      100 |      100 |      100 |                |
  IOracle.sol                |      100 |      100 |      100 |      100 |                |
  IRebasingERC20.sol         |      100 |      100 |      100 |      100 |                |
  IUnbuttonToken.sol         |      100 |      100 |      100 |      100 |                |
  IWAMPL.sol                 |      100 |      100 |      100 |      100 |                |
  IWETH.sol                  |      100 |      100 |      100 |      100 |                |
 contracts/oracles/          |      100 |       75 |      100 |      100 |                |
  ChainlinkOracle.sol        |      100 |      100 |      100 |      100 |                |
  WamplOracle.sol            |      100 |       75 |      100 |      100 |                |
 contracts/utilities/        |    71.43 |       25 |       80 |    71.43 |                |
  InstanceRegistry.sol       |    71.43 |       25 |       80 |    71.43 |          53,54 |
-----------------------------|----------|----------|----------|----------|----------------|
All files                    |       80 |    66.67 |    80.17 |    81.94 |                |
-----------------------------|----------|----------|----------|----------|----------------|

> Istanbul reports written to ./coverage/ and ./coverage.json
Error in plugin solidity-coverage: ❌ 7 test(s) failed under coverage.

Tests:

• [x] Added new unit test to confirm
• [x] yarn coverage runs properly now

Reviewers:

@Fiddlekins