Improve getting actions from the target under test

[ahumesky]

Improve getting actions from the target under test:

1. Add an keyword "mnemonic" parameter to analysistest.target_actions() to filter actions by mnemonic.

2. Add analysistest.target_action(env, mnemonic=None, output_ending_with=None) to get a single action by mnemonic or by one of its outputs.