frontend/buildcheckout.py: build default element

tests/frontend/buildcheckout.py

tests/frontend/project_default/elements/target.bst

tests/frontend/project_default/elements/target2.bst

tests/frontend/project_default/project.conf

tests/frontend/project_world/elements/compose-all.bst

...s/frontend/project_world/elements/compose-exclude-dev.bst

...s/frontend/project_world/elements/compose-include-bin.bst

tests/frontend/project_world/elements/import-bin.bst

tests/frontend/project_world/elements/import-dev.bst

tests/frontend/project_world/elements/rebuild-target.bst

...ject_world/elements/source-bundle/source-bundle-hello.bst

tests/frontend/project_world/elements/target.bst

tests/frontend/project_world/files/bar

tests/frontend/project_world/files/bin-files/usr/bin/hello

...nd/project_world/files/build-files/buildstream/build/test

...frontend/project_world/files/dev-files/usr/include/pony.h

tests/frontend/project_world/files/foo

tests/frontend/project_world/files/source-bundle/llamas.txt

tests/frontend/project_world/project.conf