Add Fraction tests: add(int), subtract(int)

The last couple (additive operations on fractions) will be exhaustive
and grueling. After all, I have to test all sorts of GCD logic. (I might
actually combine those methods into one, since subtract(Fraction)
basically delegates to add(Fraction).)

This is gonna be good.
Quick note added to Javadoc:FractionTest.testSubtractInt
Add Fraction tests: add(Fraction)

subtract(Fraction) is not tested because it delegates to
add(Fraction.negative()). testSubtractFraction has been deleted for this
reason.

These tests should prove to be exhaustive. I might go ahead and add some
tests for perfect units, etc., as well as add some cases where
denominators are not equal but not relatively prime (i.e. gcd(denom1,
denom2) != 1), but other than that I'm done with FractionTest.
FractionTest.testAddFraction: add perfect units and identity
FractionTest.testAddFraction: recode S2 to RP

I'm hoping to add some denominators that aren't relatively prime. These
will use the S2 code that I have now vacated.
FractionTest.testAddFraction: add a tiny amount of S2 non-RP tests

Just a bit, trying to test the GCD logic here. I've sprinkled these in
throughout, trying to capture a few negative fractions and both proper
and improper operations.

I can't believe I didn't have to completely turn my implementation of
Fraction inside-out fixing bugs. Worked like a charm, with reasonably
low effort. (And if add(Fraction) ever goes awry, well...let's just say
it won't go awry, shall we?)
Add S2 to Javadoc:FractionTest.testAddFraction and remove S2 TODO