Some issues with audit log testcases

audit_log testcases leave some log files behind, and may take log files from previous run into consideration if running with --repeat.

Also, audit_log_rotate is using async log write strategy, thus there is race condition between logging the workload and counting the log files.

To repeat, on e.g. 5.5:

$ ./mtr --debug-server --do-test=audit_log --repeat=10 --parallel=7
...
main.audit_log_syslog w4 [ pass ] 52
main.audit_log_old w5 [ pass ] 130
main.audit_log_rotate w7 [ pass ] 204
main.audit_log_syslog w4 [ pass ] 49
main.audit_log_old w5 [ fail ]
        Test ended at 2015-12-10 14:27:09

CURRENT_TEST: main.audit_log_old
mysqltest: At line 8: command "move_file" failed with error 1. my_errno=2

Also, audit_log_rotate uses "test_audit.log.*" glob pattern, which accepts junk suffixes (I have seen test_audit.log.%u) instead of a numeric pattern.