[land-bot] Add basic test coverage

Summary:
This patch adds some test scaffolding for land-patch.sh.
While not particularly useful yet, it will be critical for testing
rebasing until git push is successful or fails (see D7468).

Test Plan:
Verify new tests are run:

cmake -GNinja -DENABLE_SOURCE_CONTROL_TOOLS_TESTS=ON ..
ninja check-source-control-tools

Verify smoke tests run as usual:

./land-patch.sh --dry-run

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7911