Add EMIT_TEST_OPERATIONS diff flag (supports #91)

[holograph]

No description provided.

[Hubbitus]

It looks very good! Thank you!

But I could also suggest extends tests not only check test node present on patch, but it also correctly handled on apply operation by complimentary add apply test for each current test like:

    @Test
    public void testNodeEmittedBeforeReplaceOperation() throws IOException {
        JsonNode source = mapper.readTree("{\"key\":\"original\"}");
        JsonNode target = mapper.readTree("{\"key\":\"replaced\"}");

        JsonNode diff = JsonDiff.asJson(source, target, flags);

        assertEquals(2, diff.size());
        Iterator<JsonNode> iterator = diff.iterator();
        assertEquals(mapper.readTree("{\"op\":\"test\",\"path\":\"/key\",\"value\":\"original\"}"), iterator.next());
        assertEquals(mapper.readTree("{\"op\":\"replace\",\"path\":\"/key\",\"value\":\"replaced\"}"), iterator.next());
    }

    @Rule
    public ExpectedException exceptionRule = ExpectedException.none();

    @Test
    public void testApplyWithTestOnReplaceOperation() throws IOException {
        exceptionRule.expect(JsonPatchApplicationException.class);
        exceptionRule.expectMessage("[TEST Operation] value mismatch");

        JsonNode source = mapper.readTree("{\"key\":\"original\"}");
        JsonNode target = mapper.readTree("{\"key\":\"replaced\"}");

        JsonNode diff = JsonDiff.asJson(source, target, flags);

        source = mapper.readTree("{\"key\":\"changed\"}"); // Change source
        JsonPatch.apply(diff, source);
    }

It will be also very good to see context at what it failed, as I suggest in issue. So instead of just message [TEST Operation] value mismatch provide path and values, both expected and present. What are you think?

[holograph]

IMO that's covered by separate tests (the ones dealing with TEST ops). As for the error message, I'd treat that as a separate change request, though I certainly agree that more information is better...

[vishwakarma]

The changes looks good, i will merge and release the updated version.