Some partition tests are non stable

[morgo]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

go test

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

no error

3. What did you see instead (Required)

--- FAIL: TestFourReader (0.10s)
    result.go:50: 
                Error Trace:    result.go:50
                                                        partition_table_test.go:61
                Error:          Not equal: 
                                expected: "[0]\n[9]\n"
                                actual  : "[9]\n[0]\n"
                                
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,3 +1,3 @@
                                +[9]
                                 [0]
                                -[9]
                                 
                Test:           TestFourReader
                Messages:       sql:select c from pt where c < 2 or c >= 9, args:[]


--- FAIL: TestGlobalIndexDoubleRead (0.13s)
    result.go:50: 
                Error Trace:    result.go:50
                                                        partition_table_test.go:2848
                Error:          Not equal: 
                                expected: "[1 3]\n[3 4]\n[5 6]\n[7 9]\n"
                                actual  : "[3 4]\n[5 6]\n[7 9]\n[1 3]\n"
                                
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,2 +1 @@
                                -[1 3]
                                 [3 4]
                                @@ -4,2 +3,3 @@
                                 [7 9]
                                +[1 3]
                                 
                Test:           TestGlobalIndexDoubleRead
                Messages:       sql:select * from p use index (idx), args:[]

4. What is your TiDB version? (Required)

master