Allows to explicitly select a backend for the unit tests.
Currently supported are "testthat" and "tinytest".
The respective package must be installed and are loaded (but not attached).
If this function is not explicitly called, defaults to "testthat" unless
the "tinytest"'s namespace is loaded.
register_test_backend(name)[character(1)]"testthat" or "tinytest".
NULL (invisibly).