Add missing assert to chroot.clean test
Add create_temp_chroot() for testing
Add conf.dest for setting the destination path
Add more tests to test_package.py

Changes include:

* A test for package.build_source()
* A test for package.update_pkgbuild_hashes()
* Change package.build() to use exceptions and write tests.
* Add util.run_in_path_capture_stderr() and change package.build() to use it.
* Remove return booleans from changed functions.
test_repo.py: Use copy_test_project_to_temp()
test/util.py: Remove the dest argument

This function now copies the test project to the /tmp directory by default.