Run the tests specified by SELECTOR, printing results to the terminal.
SELECTOR works as described in `ert-select-tests', except if
SELECTOR is nil, in which case all tests rather than none will be
run; this makes the command line "emacs -batch -l my-tests.el -f
ert-run-tests-batch-and-exit" useful.
Returns the stats object.
(fn &optional SELECTOR)