Use pip-test-package instead of virtualenv

Thanks to #1721

Change checked out package to be pip-test-package, instead of virtualenv.

Observe old:
```
 pip ivo·ivosung ~/code/pypa/pip develop ± time py.test tests/functional/test_install.py -k test_install_using_install_option_and_editable
=============================================================================================== test session starts ================================================================================================
platform linux2 -- Python 2.7.6 -- py-1.4.20 -- pytest-2.5.2
collected 40 items 

tests/functional/test_install.py .

==================================================================== 39 tests deselected by '-ktest_install_using_install_option_and_editable' =====================================================================
===================================================================================== 1 passed, 39 deselected in 68.95 seconds =====================================================================================

real	1m9.146s
user	0m4.440s
sys	0m1.570s
```

Observe new:
```
 pip ivo·ivosung ~/code/pypa/pip speedup_test_editable ± time py.test tests/functional/test_install.py -k test_install_using_install_option_and_editable
=============================================================================================== test session starts ================================================================================================
platform linux2 -- Python 2.7.6 -- py-1.4.20 -- pytest-2.5.2
collected 40 items 

tests/functional/test_install.py .

==================================================================== 39 tests deselected by '-ktest_install_using_install_option_and_editable' =====================================================================
===================================================================================== 1 passed, 39 deselected in 3.93 seconds ======================================================================================

real	0m4.132s
user	0m2.397s
sys	0m0.413s
```