message: tidy up test matchers, add comment
diff: import {createMatchers, createSerializer} from 'jest-emotion'
  import * as emotion from 'emotion'
  -import * as systemProps from 'styled-system'
  +import {styles as systemProps} from 'styled-system'
  expect.extend(createMatchers(emotion))
  expect.addSnapshotSerializer(createSerializer(emotion))
  const stringify = d => JSON.stringify(d, null, ' ')
  +/**
  + * These are props that styled-system aliases for backwards compatibility.
  + * For some reason, they don't show up in our toImplementSystemProps() matcher,
  + * so we skip over them.
  + */
  const ALIAS_PROP_TYPES = ['w', 'align', 'justify', 'wrap']
  expect.extend({
  