Modified toString() for denom=1. Test output...
Now returns just the numerator as a String if the denominator is equal
to 1. Output of tests:

$ 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.
test "Add: that 1/2 + 1/4 gives 3/4" passes.
test "Add: that 2/9 + 3/5 gives 37/45" passes.
test "Subtract: that 2/9 - 3/5 gives -17/45" passes.
test "Divide: that (2/9) / (3/5) gives 10/27" passes.
test "AbsValue: that absValue(-7/9) = 7/9" passes.
test "Negate: that Negate(7/9) = -7/9" passes.
test ".toString(): that (6/5).toString() is equal to "6/5" " passes.
test ".toString(): that (6/1).toString() is equal to "6" " passes.
FractionTest: All tests pass