handleSliderTouchStart/Move/End unit tests

Although there is a separate PR for updateSelection unit tests, this PR includes includes updateSelection unit tests as the updateSelection patch has not yet been landed. Therefore, the reviewer of this patch can ignore the updateSelection unit tests as part of reviewing this patch.
Sorry, I forgot to read the comment here at the previous "updateSelection" unit tests. It not quite good to chain the unit test because the execution sequence is decided by test agent. Once they change it, the chained test may fail to execute. So, as to these tests, I would suggest to merge them into previous tests. You may do one updateSelection and test both selectedFileNames and UI state.