Improve 3 tests

[philipbutler]

Most of the tests are too basic (a = [0, 1, 2, 3], a = [1, 1, 1, 1]) which causes some wrong solutions to pass.

For example, it's intended that a gets read from, but if one peeks at the expected result, they can avoid reading altogether by writing out[_thread_index_] = thread_index.

To try and improve the first 3 tests, I used a numpy random integer generator, populating a with values 0-9.

If this is a good proposal and is accepted, I would be happy to update the other tests.

Thanks for making a great intro to GPU programming :)