bst-init: Ensure --element-path is respected by the command

Previously "elements" was hard-coded as the path for the elements
directory whereas it was supposed to be configurable via the
`element_path` option to `init_project()`. This led to incorrect
behavior when `bst init` was run `--element-path` option.

Also, extend tests to test the creation of elements directory.

Fixes #398.
2 files changed