Software Testing Knowledge

Here are some points to keep into mind:
In general, Agile development works with relatively few, very briefly-defined requirements in the form of user stories. Agile’s requirements usually are in the form of user stories, which by definition are very brief and relatively few in the number in keeping with Agile’s focus on just the work to be done in the near term. User story acceptance tests also tend to be limited to the small user story, which is closest in scope to integration tests.
Agile tends not to devote much attention to software design. Instead Agile usually limits its attention to the individual small pieces of code implementing particular features, often without necessarily addressing the design--how those features fit together with each other and with other components. Consequently, typical agile development often lacks system testing. Not surprisingly, such lack of design can cause agile development upstream difficulties when the individual features fail to work together adequately overall.

I recommend also to read the following article on this topic

What is Agile Software Testing?