test(chain): add several basic scenarios to test transaction conflicts

[pluveto]

Description

This update introduces several test cases to validate the transaction conflict handling logic within the TxGraph. The tests encompass a range of scenarios including:

1. Simple cases with isolated transactions (only A exists).
2. Scenarios with conflicting transactions (B and B' spending A) under two basic conditions:
   • One of the conflicting transactions is confirmed.
   • Conflicts are resolved based on the last_seen timestamp.

Changelog Notice

• Added unit tests for transaction conflict resolution in TxGraph.

Checklists

All Submissions:

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

[LagginTimes]

Thank you for your contribution! The test case involving two conflicting transactions with different last_seen values is already addressed by the existing test scenario: "2 unconfirmed txs with same last_seens conflict." Other than that, the rest of the tests look good to me.

[pluveto]

OK. I've removed this redundant case.

[LagginTimes]

Could you please squash the second commit into the first one to keep the commit history clean? Everything will look good to me once that's done. Thanks!