Unit testing for concatenating two int arrays
Testing


concat isn't a great name for a test, it doesn't tell me anything about what it is that the test does, it simply copies the name of the method being called.  Something like concatShouldReturnConcatenationForTwoPopulatedArrays might be a better description
Personally, for a test this simple, I'd tend to define the expected results for the test within it rather than calculating it on the fly.  This makes it less likely that you'll get a failure because your test is wrong.  It also allows you to use the junit array comparison.

Assert.assertArrayEquals( expectedResult, result );

You've only got one test, I'd tend to add at least 3 more for:

concat([], numbers2)
concat(numbers1, [])
concat([], [])