unstable test TestLFUFreshMemUsage

[hawkingrei]

Bug Report

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1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

=== RUN   TestLFUFreshMemUsage

    lfu_cache_test.go:60: 

        	Error Trace:	statistics/handle/cache/internal/lfu/lfu_cache_test.go:60

        	Error:      	Not equal: 

        	            	expected: 60

        	            	actual  : 52

        	Test:       	TestLFUFreshMemUsage

--- FAIL: TestLFUFreshMemUsage (0.00s)

4. What is your TiDB version? (Required)

[tiancaiamao]

It's still unstable ... or, it fails stably

cd statistics/handle/cache/internal/lfu
go test -run TestLFUFreshMemUsage
--- FAIL: TestLFUFreshMemUsage (0.00s)
lfu_cache_test.go:81:
Error Trace: /home/genius/project/src/github.com/pingcap/tidb/statistics/handle/cache/internal/lfu/lfu_cache_test.go:81
Error: Not equal:
expected: 0x30
actual : 0x0
Test: TestLFUFreshMemUsage
FAIL
exit status 1
FAIL github.com/pingcap/tidb/statistics/handle/cache/internal/lfu 0.018s

[tiancaiamao]

On master barnch, it need the --tags intest flag to run.