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.

[LagginTimes]

Would it be valuable to include tests for conflict scenarios as well? For example:

let mut map1: BTreeMap<i32, &str> = BTreeMap::new();
map1.insert(1, "a");
let mut map2: BTreeMap<i32, &str> = BTreeMap::new();
map2.insert(1, "b");

map1.merge(map2);

assert_eq!(map1.len(), 1);
assert_eq!(map1.get(&1), Some(&"b"));

[pluveto]

I've added refactored empty check assertions and merge conflict tests. Squashed.

[evanlinjin]

ACK 871df7bb24b6c15a096d6b98bdfc7d6877cbd29c

[LagginTimes]

ACK 665273d