Avoid using '/tmp' directory in our unit tests

I'm used to run unit tests on a remote server before summit a large patch because sometimes a testcase failure can not be reproduced locally.

Recently yangzhe1991 and me share a remote server with a different account. We found that some of the unit tests write to '/tmp' with a fixed name and do not clean the file after test. This cause that the unit test can only be succeeded for one person...

Open an umbrella issue to address this.