See #75 and #72 for context.
The envsitepackagesdir test was broken, the attached patch fixes that and a problem with lazy-loading properties (which also needs to be done on the getargvlist → _processcommand code path).
the test does not really test the problem.
The test doesn't pass without the fix in the same commit.
Do you want something more low-level as well?
Fixed in #92. For the record, the code was correct at the time you merged it, the test wasn't.
This issue has been moved to GitHub: https://github.com/tox-dev/tox/issues/90