Throw exception given 0 divisor. Test output ...
Now first test is a pass, correctly get an exception.

$ java -ea FractionTest
FractionTest tests verbose output:
test "That 1/0 should throw an IllegalArgumentException exception" passes.
test "That multiplying 1/2 by 3/5 gives 3/10" passes.
test "That 1/2 .equals 1/2" passes.
test "That 1/2 .equals 3/6" passes.
test "That -1/2 .equals 1/-2" passes.
test "That -1/-2 .equals 1/2" passes.
test "That 4/-8 does not .equals 1/2" passes.