test(core): add unit tests for merge trait

[pluveto]

Description

This update adds unit tests for the Merge trait implementations on BTreeMap, BTreeSet, Vec, and tuples. It also includes tests for the is_empty and take methods to ensure their correct functionality.

Notes to the Reviewers

• The tests cover basic scenarios for merging collections and checking their emptiness.
• Consider the potential method name conflict with BTreeSet's existing take method.

Changelog Notice

• Added unit tests for merging functionality and utility methods (is_empty, take) on various collection types.

Checklists

All Submissions:

• I've signed all my commits.
• I followed the contribution guidelines.
• I ran cargo fmt and cargo clippy before committing.

[ValuedMammal]

Should we qualify these with the trait method also?

Suggested change

	assert!(vec.is_empty());
	assert!(Merge::is_empty(&vec);