Improve the clarity of unit test fixtures

The unit test fixtures are a bit messy at the moment, the performance is not good as well. Some fixtures to create app object and step objects seems redundant. We need to set the app as a session with a clear tear-down process to reset, instead creat a new object and setting parameters from the beginning.

@superstar54 let's try to set a pair coding session between two of us to tidy things up.