Make results details in ert-run-tests-batch configurable

Make results details in ert-run-tests-batch configurable

* lisp/emacs-lisp/ert.el (ert-batch-print-length)
(ert-batch-print-level,.ert-batch-backtrace-line-length)
(ert-batch-test, ert-run-tests-interactively): Added the three
variables, bound them to these settings when formatting batch
test results including backtraces. Removed the optional
parameters output-buffer & message-fn from
ert-run-tests-interactively.
* test/lisp/emacs-lisp/ert-tests.el
(ert-test-run-tests-interactively, ert-test-run-tests-batch): use
cl-letf to capture output, new tests resp.
* test/lisp/ert-x-tests.el (ert-test-run-tests-interactively-2):
Changed to use cl-letf to capture output instead of using
message-fn.
* lisp/emacs-lisp/backtrace.el (backtrace--line-length-or-nil)
(backtrace--print-func-and-args): Fixed a bug when setting
backtrace-line-length to nil by adding a new function to check
for that case & having backtrace--print-func-and-args use it.
* doc/misc/ert.texi: document the new variables & their usage
(bug#51037).