selftests: kselftest framework: add error counter

Some tests track errors in addition to test failures. Add ksft_error
counter, ksft_get_error_cnt(), and ksft_test_result_error() API to
get the counter value and print error message.

Update ksft_print_cnts(), and ksft_test_num() to include error counter.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>