Java: grade-school: use assertEquals instead of assertThat

[FridaTveit]

In the grade-school tests assertThat is used for the test assertions. To be more consistent with the other exercises we should use assertEquals instead. For example:

assertThat(school.roster(), is(expected));

Should be changed to:

assertEquals(expected, school.roster());

[AndresAcuna]

Changed for consistency, let me know if it needs more changes.

[yuval105]

I'll go for it.

[FridaTveit]

@yuval105 Thanks for wanting to help but in this case @AndresAcuna got there first 🙂 Feel free to work on any other issue or anything else you think needs improving! 🙂

[sjwarner-bp]

Thanks for getting this done @AndresAcuna - closing now as fixed in #1591