ztest: Don't clear test stats before running suite

test_main can call ztest_run_all multiple times. For example
tests/bluetooth/host/keys/bt_keys_get_addr/src/main.c does so.  If the
first call skips some tests and then they run in the second call,
everything is fine. However if they passed or failed in the first run
and skipped in the second run, then the test output (json, xml, and
printed summary) show the test as skipped even though it ran.

The solution is to not clear the test stats.

__ztest_init_unit_test_result_for_suite was added in commit
996c8457 by nashif.

Signed-off-by: Jeremy Bettis <jbettis@google.com>