Test self.copy() return value when packaging - #7389

Changelog: Feature: Document return value of self.copy() in the package() method.
Docs: conan-io/docs#1773

• Refer to the issue that supports this Pull Request: closes [feature] self.copy() prints warning if file doesn't exist #7347
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

^{Note: By default this PR will skip the slower tests and will use a limited set of python versions. Check here how to increase the testing level by writing some tags in the current PR body text.}