Add a test suite

### 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](https://github.com/pypa/cibuildwheel/pull/2590#issuecomment-3412479167) contains some code from cibuildwheel that does the sort of testing that should be performed for xvenv.

Start by reviewing the xbuild and xvenv interfaces and the testing code linked from the cibuildwheel pull request. Define coverage for both interfaces and establish what successful test execution should verify; the issue does not name files, entry points, or specific cases.

Written by the indexing model from the issue text.