flaky test in util/gctuner

[hnes]

Bug Report

1. Minimal reproduce step (Required)

$go test -count=10 -run Test* github.com/pingcap/tidb/util/gctuner
--- FAIL: TestGlobalMemoryTuner (2.22s)
    memory_limit_tuner_test.go:103:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:103
                Error:          Not equal:
                                expected: 858993459
                                actual  : 1181116006
                Test:           TestGlobalMemoryTuner
    memory_limit_tuner_test.go:64:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:64
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/panic.go:522
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/testing.go:871
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:103
                Error:          Should be false
                Test:           TestGlobalMemoryTuner
--- FAIL: TestTuner (0.29s)
    tuner_test.go:57:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/tuner_test.go:57
                Error:          "285" is not less than or equal to "250"
                Test:           TestTuner
--- FAIL: TestGlobalMemoryTuner (1.67s)
    memory_limit_tuner_test.go:92:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:92
                Error:          Not equal:
                                expected: 1181116006
                                actual  : 858993459
                Test:           TestGlobalMemoryTuner
    memory_limit_tuner_test.go:64:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:64
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/panic.go:522
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/testing.go:871
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:92
                Error:          Should be false
                Test:           TestGlobalMemoryTuner
--- FAIL: TestGlobalMemoryTuner (1.66s)
    memory_limit_tuner_test.go:92:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:92
                Error:          Not equal:
                                expected: 1181116006
                                actual  : 858993459
                Test:           TestGlobalMemoryTuner
    memory_limit_tuner_test.go:64:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:64
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/panic.go:522
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/testing.go:871
                                                        /x/tarball/gctuner/tidb-master/util/gctuner/memory_limit_tuner_test.go:92
                Error:          Should be false
                Test:           TestGlobalMemoryTuner
--- FAIL: TestFinalizer (0.01s)
    finalizer_test.go:40:
                Error Trace:    /x/tarball/gctuner/tidb-master/util/gctuner/finalizer_test.go:40
                Error:          Not equal:
                                expected: 9
                                actual  : 8
                Test:           TestFinalizer

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[hnes]

Refer tikv/pd#5953.