First end2end tests using the examples/ folder

examples/ should be useful for a new developer wanting to use this project. Due to this nature, many of the tests in that folder should be naturally failing, for example to show shrinking at work:
```
1) ListConcatenationTest::testRightIdentityElement
Summing 42 to 0
Failed asserting that 43 matches expected 42.

/home/giorgio/code/eris/examples/SumTest.php:24
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:48
/home/giorgio/code/eris/src/Eris/Quantifier/RoundRobinShrinking.php:45
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:26
/home/giorgio/code/eris/src/Eris/Quantifier/Evaluation.php:50
/home/giorgio/code/eris/src/Eris/Quantifier/ForAll.php:28
/home/giorgio/code/eris/examples/SumTest.php:25
```

By running those tests in an external process, this end2end test opens the way to make assertions on the state of the example/ tests:

* SampleTest should be passing in its entirety
* ListConcatenationTest::testRightIdentityElement should fail
* ListConcatenationTest::testRightIdentityElement by showing this error message