Clean up mission tests.

Add default value for mission title in factory girl and clean up indentation.
Reduce duplication in test code by pulling common lines to before blocks.
Add tests for invalid mission title and description.

Stop using 'should' in test descriptions.
Remove extra lines.
Uncomment old error message case and change it to a pending test.
2 tests checked for the same condition. These were combined into 1 test.
Usage of an undefined variable 'title' might cause unpredictable results in future.