test: flush redis for rate limiting tests

[beardnick]

Description

Which issue(s) this PR fixes:

This is a separate PR from #12751. That PR is too large to review, so I decided to split it into several smaller PRs. This particular PR focuses solely on improving the stability of tests for rate-limiting plugins that require Redis.

Notice

To use code like this, I had to adjust some cases to ensure the test results were independent.

    $extra_init_worker_by_lua .= <<_EOC_;
        require("lib.test_redis").flush_all()
_EOC_

For example:
The rate limit here is 2 requests per minute. Previously, there was no cleanup process between tests, so TEST 5 depended on TEST 4. TEST 4 increased the counter by 1, which made TEST 5 expect [200, 503, 503]. My PR introduced a cleanup process for each test case, so now TEST 5 expects [200, 200, 503]. I believe having independent test cases is more stable and easier to understand.

apisix/t/plugin/limit-count-redis-cluster.t

Lines 167 to 180 in c0c906e

	=== TEST 4: up the limit
	--- request
	GET /hello
	--- error_log
	try to lock with key route#1#redis-cluster
	unlock with key route#1#redis-cluster
	=== TEST 5: up the limit
	--- pipelined_requests eval
	["GET /hello", "GET /hello", "GET /hello"]
	--- error_code eval
	[200, 503, 503]

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

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[beardnick]

The failed tests were unrelated to my PR. All the rate limiting tests passed successfully. @Baoyuantop