Test runner: print out list of failures at end

Previously the test results summary looked like:
```
--- 6 tests skipped.
['Tests/test_image_point.py', 'Tests/test_imagegrab.py', 'Tests/test_imageqt.py', 'Tests/test_imagetk.py', 'Tests/test_locale.py', 'Tests/test_numpy.py']
*** 1 test of 94 failed.
```
https://travis-ci.org/python-imaging/Pillow/jobs/22731574

The summary now looks like:
```
--- 4 tests skipped:
test_imagegrab, test_imageqt, test_imagetk, test_locale
*** 1 test of 96 failed:
test_mode_i16
```
https://travis-ci.org/hugovk/Pillow/jobs/22753300

Or when passing:
```
--- 5 tests skipped:
test_imagegrab, test_imageqt, test_imagetk, test_locale, test_numpy
95 tests passed.
```
https://travis-ci.org/hugovk/Pillow/jobs/22753803