Simple example of test doubles using Mockito and Junit5 with Java

FootballTeamClassTest.java file represents some example usages for different kinds of test doubles. Class has the following test doubles:

- Dummy: Dummy football player is created and used as an input parameter.
- Stub: The function is stubbed to return the wanted values, when it is called.
- Mock: Test is checking if the logScorePlayer method is called within the FootballTeam class when the scoreGoal function is called.
- Spy: Here Spy allows us to track the interactions with the object and also offering access to the methods of the object.