This commit adds a test case for the following scenario:
 1. The first few publications succeed with err == nil.
 2. One of the txs is mined.
 3. Subsequent publications start failing because they are using the
    same nonce as the tx mined in (2).

This edge case is currently overlooked in the txmgr, and will cause the
shouldAbortImmediately method to return true and not wait for the
appropriate confirmations.