red-black tree: store keys in the unit test

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
red-black tree: use a cache for the node with the lowest key

Link: https://github.com/GeneAssembly/biosal/issues/701
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>