- convert margin left to display flex instead

- NOTE: possibly revert this commit when data grid borders modifiers are converted to Emotion - I'd rather handle this at the topmost `.euiDataGrid` level

- we're going to replace this with individual per-control Emotion styles

+ convert Enzyme tests to RTL
+ only test render snapshot on React 18 - all other tests run on react 16/17 and should more than suffice to validate that it works there
+ remove valid droppable area test - this one is ludicrous to try and test in a jsdom/unit test

+ reduce extra div wrapper

+ update datagrid enzyme tests to account for emotion wrappers

+ improve responsive styles by moving to `auto` column width

- just inline it in the component, since it's super minor

+ update Enzyme tests to RTL, snapshot full popover