test: Bitcoin outbound code coverage improvement

[ws4charlie]

Describe the Issue
From what we've seen in the previous Bitcoin refactor PR: #3381, the code relating to Bitcoin outbound and signer are not well covered by unit tests and some of them needs improvement, for example:

• The TryProcessOutbound is not covered by unit test.
• The SelectUTXOs has a test flag purely for unit test, we could remove it to be clean.
• The can be other important methods not covered as well.
• It is good practice to return an error from TryIncludeOutbound method, even if it's ignored by caller.

Expected Outcome
A description of what outcome you are seeking.

[ws4charlie]

@swift1337 TryIncludeOutbound is mentioned above for your comment #3381 (comment)