Allow local testing

- Move integration testing to run locally And not in Docker. This patch is to prove things work, so I've just moved one test across (though most of the changes are in test setup that can be re-used across tests). If this gets tractions, more tests could be moved across. Reasons for this approach: * Simplicity: tests have no external requirements besides `git` * Debugability: running locally you can spin up a debugger (or just sleep in tests) and inspect the running environment. This is possible with Docker, but more of a hassle * Reproducibility: tests are decoupled from the user's environment, so failures should be reproducible This is achieved via: * Using git's worktree's[1] to run things like `git commit` without modifying the working repo * Having the tests build a strongbox binary that we can then configure `git` with (built once per run) * Adding some test setup with actual keys etc. rather than relying on the user having keys define For posterity, the commands to setup the keys etc go run . -identity-file ./testdata/.strongbox_identity -gen-identity test-key grep --only-matching --perl-regexp '(?<=# public key: ).*' ./testdata/.strongbox_identity > ./testdata/.strongbox_recipient Link: https://git-scm.com/docs/git-worktree [1] - Add run of local tests in GitHub