Expand PHPUnit test coverage of the Diff component

Try this on.

CRAP scores:

DiffOp 132 -> 26

HWLDFWordAccumulator also 132 -> 26

DiffEngine 12,210 -> 4,318

I only tested public methods using no mocking. Some of the higher complexity public methods look like they might not be used by core. Diff::check(), in fact, says it's only for debugging purposes.

Also verified that patch #76 in #2337283: Add a composer.json file to every component requires drupal/utility for this component.

Some of the higher CRAP score methods really need some refactoring. That's only possible if we test them. ::gulp::

Typo. Amazing how PHPUnit can find it but run-tests.sh can't. :-)

1. +++ b/core/tests/Drupal/Tests/Component/Diff/Engine/DiffEngineTest.php
   @@ -0,0 +1,89 @@
   +      [[], [], []],
   ...
   +      [
   ...
   +      [
   ...
   +      [
   

   Let's use string keys to explain the different testcases better

2. +++ b/core/tests/Drupal/Tests/Component/Diff/Engine/DiffOpTest.php
   @@ -0,0 +1,36 @@
   +   * @covers ::reverse
   +   * @expectedException \PHPUnit_Framework_Error
   +   */
   

   Note: The recommended way is now to not use the annotation but rather the setExpectedException method, sorry

#8.1: Not sure what you mean. Read the test to see what it does.

#8.2: Doesn't make any sense, but OK.

#8.1: Not sure what you mean. Read the test to see what it does.

This is entirely not the point. The point is having a MUCh easier time to quickly scan it, for example when you have a test failure.
PhpUnit is showing those keys additional.

+++ b/core/tests/Drupal/Tests/Component/Diff/Engine/DiffEngineTest.php
@@ -31,11 +31,11 @@ class DiffEngineTest extends \PHPUnit_Framework_TestCase {
+      'empty' => [[], [], []],

OK, gotcha. +1

Now who will RTBC since you patched it? :-)

Come on, I just changed array keys :)

Committed/pushed to 8.1.x and cherry-picked to 8.0.x. Thanks!