Add more tests

[adorade]

Add more tests or update the existing ones. Since I might overlook something, I want to ensure that everything functions correctly.

For testing purposes, I will be using only Gulp v5.

[adorade]

Add tests for the failAfterError option when it is not configured (default is true), and when it is explicitly set to true or false.

• should not fail after emitting no errors
• failAfterError is not configured
• failAfterError is set to true
• failAfterError is set to false

See #29

[adorade]

Plugin Functionality

• should not emit an error on buffered file
• should ignore file
• should fix the file without emitting errors

Fail after error

• should FAIL after emitting an error if failAfterError is not configured
• should FAIL after emitting an error if failAfterError is set to true

Sourcemap Handling

• should emit no errors when stylelint rules are satisfied
• should apply sourcemaps correctly when using a custom sourcemap file
• should ignore sourcemaps with no sources