Automatic merge from submit-queue (batch tested with PRs 49133, 51557, 51749, 50842, 52018)

enhance unit tests of advance audit feature

This change addresses comments from @crassirostris 
https://github.com/kubernetes/kubernetes/pull/49115#discussion_r133416080

It does three things:
1. use auditinternal for unit test in filter stage
2. add a seperate unit test for Audit-ID http header
3. add unit test for audit log backend


**Release note**:
```
NONE
```