Update/reinforce mempool tests

The following discussions from !6787 (merged) should be addressed:

• @ACoquereau started a discussion:

  Since this MR updates this test, could it be the good time to revamp it?

• @ACoquereau started a discussion:

  Same as test_get_post_mempool_filter, could it be the good time to revamp this test?

• @ACoquereau started a discussion:

  A test to check the full (in size) mempool should be added in another MR?

Also:

• The full mempool tests should involve non-manager operations, and should check that the error message for is different when the mempool if full with them (since no fees are enough to replace them).
• Test adding an operation larger than max_total_bytes.
• Test conflict error with non-manager operations.
  • Manager / drain_delegate conflict: check error message