You can pull in a local project (e.g. for testing changes) using either vcs or path repository types in Composer:
Put the path to the local project VCS (Git, Hg, SVN etc) in the url field.
See: composer_vcs.json
Put the relative path to the local project in the url field.
See: composer_path.json
Prefix the branch name you want to use with dev-, so my-branch becomes dev-my-branch.
Install a Local Package with Composer