flaky test TestCachingSha2CacheTLS/TestCache

[lance6716]

--- FAIL: TestCachingSha2CacheTLS (0.44s)
    --- FAIL: TestCachingSha2CacheTLS/TestCache (0.40s)
        caching_sha2_cache_test.go:163:
            	Error Trace:	/home/linux1/go-mysql/server/caching_sha2_cache_test.go:163
            	Error:      	"107" is not less than "50"
            	Test:       	TestCachingSha2CacheTLS/TestCache

[lance6716]

another case

--- FAIL: TestCachingSha2CacheTLS (0.33s)
    --- FAIL: TestCachingSha2CacheTLS/TestCache (0.28s)
        caching_sha2_cache_test.go:163:
            	Error Trace:	/home/linux1/go-mysql/server/caching_sha2_cache_test.go:163
            	Error:      	"68" is not less than "50"
            	Test:       	TestCachingSha2CacheTLS/TestCache

These cases are found in s390x daily CI