feat(assert): assert cookie exists

[RomainLanz]

Hey! 👋

Proposed changes

This PR let us assert that a cookie or a plainCookie exists without knowing it's value.

This is useful in some case like verifying that the cookie of the session exists after logging in.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works.
• I have added necessary documentation (if appropriate)

[coveralls]

Coverage increased (+0.2%) to 80.83% when pulling fb11f37 on RomainForks:feature/assert-cookie-exists into 413090f on adonisjs:develop.