Improve test mocking by adopting sinon

[ryanluker]

https://github.com/ryanluker/vscode-coverage-gutters/pull/320#discussion_r650550183 @mattseddon FYI from our discussion in the other PR, I will work on plugging away at fixing and refactoring the existing manual mocks.

Background

See the past discussion and examples section but this issue will track the refactoring and removal of the manual mocks from the test suite. While useful at the beginning when the codebase and test amount was small, they have not scaled well and it is apparent we need a better tool.

Prep Work

• [x] Update mocha to v9
• [x] Add sinon-chai https://www.chaijs.com/plugins/sinon-chai/
• [x] Convert fakeConfig into an applicable sinon mock
• [x] Remove deprecated assert.equal usage assert.equal

  /** @deprecated since v9.9.0 - use strictEqual() instead. */
  function equal(actual: any, expected: any, message?: string | Error): void;

Refactor existing manual mocks

• [x] test\coverage-system\coverage.test.ts
• [x] test\coverage-system\coverageservice.test.ts
• [x] test\extension\config.test.ts
• [x] test\files\coverageparser.test.ts
• [x] test\files\filesloader.test.ts

Past discussions and examples around mocking

https://github.com/ryanluker/vscode-coverage-gutters/pull/270 https://github.com/ryanluker/vscode-coverage-gutters/pull/218 https://github.com/ryanluker/vscode-coverage-gutters/issues/198