unit tests: add a project plan document

unit tests: add a project plan document

In our current testing environment, we spend a significant amount of
effort crafting end-to-end tests for error conditions that could easily
be captured by unit tests (or we simply forgo some hard-to-setup and
rare error conditions). Describe what we hope to accomplish by
implementing unit tests, and explain some open questions and milestones.
Discuss desired features for test frameworks/harnesses, and provide a
comparison of several different frameworks. Finally, document our
rationale for implementing a custom framework.

Co-authored-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>