Add a test suite #21
Description
What is the problem or limitation you are having?
At present, xbuild has no tests.
Describe the solution you'd like
We should add a test suite that exercises xbuild and xvenv interfaces.
Describe alternatives you've considered
No real option - tests are non-optional.
Additional context
This PR contains some code from cibuildwheel that does the sort of testing that should be performed for xvenv.