The software test plan in project development using agile is different to the plan used in traditional development methodologies (example: WaterFall).
The tester usually reads the requirements and defines the test plan but the ideology of agile project is creating more functionality instead of documentation. Therefore, the existing documentation may not be enough and the tester needs to communicate with the developer. An objective of the agile development is greater communication between people involved in the project.
Another problem is the timeline; the development is executed by sprint. The sprint is usually done in a short time and it is necessary to plan, develop and test the functionality. The functionality is only considered implemented when tested. In an ideal system the development of the test should be the same as the testing time, but when the development is delayed, the time test is compressed.
In agile development, it is possible for the test not to be executed by an independent team but by any developer who is given the role of tester of certain functionality. The only restriction is that the developer has no involvement in that functionality of the test. The approach of not having an independent team for the execution of the test has the advantage that the developer becomes more sensitive to the testability of the code. In other words, when the developer produces code, he remembers details necessary to allow the application to be tested.
The agile testers have ten principles; they are represented in the image below.
I recommend the book Agile Testing: A Practical Guide for Testers and Agile Teams.
