Use a smaller repository in the tests for speed.

[alex]

Refs #1721

[alex]

For comparison:

Before:

$ py.test tests/functional/test_install_upgrade.py -ktest_upgrade_vcs_req_with_dist_found                              🕒 2.44s
==================================================== test session starts =====================================================
platform darwin -- Python 2.7.5 -- py-1.4.20 -- pytest-2.5.2
collected 20 items

tests/functional/test_install_upgrade.py .

============================== 19 tests deselected by '-ktest_upgrade_vcs_req_with_dist_found' ===============================
========================================== 1 passed, 19 deselected in 40.12 seconds ==========================================

After:

$ py.test tests/functional/test_install_upgrade.py -ktest_upgrade_vcs_req_with_dist_found
==================================================== test session starts =====================================================
platform darwin -- Python 2.7.5 -- py-1.4.20 -- pytest-2.5.2
collected 20 items

tests/functional/test_install_upgrade.py .

============================== 19 tests deselected by '-ktest_upgrade_vcs_req_with_dist_found' ===============================
========================================== 1 passed, 19 deselected in 4.14 seconds ===========================================