tests: make TestIndexLookupMergeJoinHang and TestIssue18068 stable (#23741)
tests: make TestIndexLookupMergeJoinHang and TestIssue18068 stable (#23741)

 
kyle-wang/tidb,ngaut/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,ngaut/tidb,kyle-wang/tidb,c4pt0r/tidb,kyle-wang/tidb,kyle-wang/tidb,coocood/tidb,c4pt0r/tidb,c4pt0r/tidb,ngaut/tidb,ngaut/tidb,c4pt0r/tidb,pingcap/tidb,c4pt0r/tidb,coocood/tidb,pingcap/tidb