Refactor testing routine

__tests__/helpers/fileReader.ts

__tests__/helpers/index.ts

__tests__/html-gmail-gmail.test.ts

__tests__/html-gmail-outlook.test.ts

__tests__/html-outlook-gmail.test.ts

__tests__/html-outlook-outlook.test.ts

__tests__/htmltext-gmail-gmail.test.ts

__tests__/htmltext-gmail-outlook.test.ts

__tests__/htmltext-outlook-gmail.test.ts

__tests__/htmltext-outlook-outlook.test.ts

__tests__/plain-gmail-gmail.test.ts

__tests__/plain-gmail-outlook.test.ts

__tests__/plain-outlook-gmail.test.ts

__tests__/plain-outlook-outlook.test.ts

__tests__/utils.ts

jest.config.js