Unbreak test_detect_duplicates.py and link into :all.

I think the test could be improved to be more black box and not dip
in to test a private method directly, but this fixes the basic break
due to a signature change in the tested private method.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python:: --test-pytest-options="-v -ktest_detect_duplicates"
...
00:15:23 00:01   [test]
00:15:23 00:01     [pytest]
00:15:23 00:01       [run]
                     ============== test session starts ===============
                     platform linux3 -- Python 2.6.8 -- py-1.4.20 -- pytest-2.5.2 -- /home/jsirois/dev/3rdparty/pyenv/versions/2.6.8/bin/python
                     plugins: cov, timeout
                     collected 282 items

                     tests/python/pants_test/tasks/test_detect_duplicates.py:68: DuplicateDetectorTest.test_duplicate_found PASSED
                     tests/python/pants_test/tasks/test_detect_duplicates.py:73: DuplicateDetectorTest.test_duplicate_not_found PASSED
                     tests/python/pants_test/tasks/test_detect_duplicates.py:78: DuplicateDetectorTest.test_fail_fast_error_raised PASSED

                      279 tests deselected by '-ktest_detect_duplicates'
                     ==== 3 passed, 279 deselected in 0.83 seconds ====

00:15:25 00:03     [junit]
00:15:25 00:03     [specs]
               SUCCESS

Bugs closed: 204

Reviewed at https://rbcommons.com/s/twitter/r/484/