test: old test case is unstable, running fail sometimes.

[membphis]

What this PR does / why we need it:

as title.

In test 3 of the old use case, both the old and new configurations may trigger errors and generate logs, and they are random. There should be only one error at a time, which makes it easier to ensure stable testing.

unstable output: https://github.com/apache/incubator-apisix/runs/921554160?check_suite_focus=true#step:6:121

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible?