support testing the sdist of a project
This issue is to add support for testing the "sdist" version of a project in addition to testing an install.
The reason for this request is that a source checkout may include more tests than what's included in the installed version. For example, a source checkout may include tests of certain functionality in setup.py (which doesn't itself get installed), or other types of extra tests (e.g. doctests of documentation files which don't get installed).
It's possible that this feature is already supported, or that there are good reasons that this needn't be supported.