fix: replace 'should' dependency with native assertions in cookies module

The 'should' module was listed as a devDependency but was required by
lib/cookies/assertion.js at runtime. This caused 'Cannot find module should'
errors for users installing supertest.

This fix replaces all uses of the 'should' assertion library with native
JavaScript assertion helper functions:
- assertHasProperties: checks if object has specified properties (array or object)
- assertNotHasProperties: checks if object does not have specified properties
- assertEqual: checks strict equality
- assertNotEqual: checks strict inequality

The native implementations match the behavior of should.js, including the
'false negative fail' behavior when checking not.have.properties with empty
arrays/objects.

Fixes: https://github.com/forwardemail/supertest/issues/875#issuecomment-3713828994