test: Add jest-enzyme assertion library for better frontend tests

[suddjian]

SUMMARY

The jest-enzyme library adds lots of useful assertions onto jest. This PR improves frontend test readability and adds useful context to failure messages.

An example of a test failure using an enzyme assertion:

FAIL  spec/javascripts/addSlice/AddSliceContainer_spec.tsx (60.221 s)
  ● AddSliceContainer › renders a select and a VizTypeControl

    Expected "Memo(StyledSelect)" not to exist. Instead found 1 nodes.
    Found Nodes: <Memo(StyledSelect) clearable={false} ignoreAccents={false} name="select-datasource" onChange={[Function: bound changeDatasource]} options={{...}} placeholder="Choose a datasource" style={{...}} value={[undefined]} width={600} />

      50 |
      51 |   it('renders a select and a VizTypeControl', () => {
    > 52 |     expect(wrapper.find(Select)).not.toExist();
         |                                      ^
      53 |     expect(wrapper.find(VizTypeControl)).toExist();
      54 |   });
      55 |

      at Object.<anonymous> (spec/javascripts/addSlice/AddSliceContainer_spec.tsx:52:38)

[nytai]

lgtm!