TxPool test suite must cover success cases and failure cases

[bvrooman]

Currently, the TxPool test suite covers:

• Simple success cases (simple insertion, simple dependency chains)
• Most error cases

The TxPool test suite does not cover:

• Complex dependency chains
• All error cases (there are more Error variants than tests)
• Stress testing (high volume, high concurrency, etc.)
• Benchmarking

[xgreenx]

Transaction Pool V2 improves the test coverage #2193

[AurelienFT]

I think this can still be open as I improved a bit but mainly reordered and simplified and there is points like benchmarks that I don't have and should be done.