run prettier on failing tests for toMatchFormattedCss

This will result in better diffs because diffs are typically per block
and/or per line. But lightningcss will simplify certain selectors and
the diff won't be as clear.

We will only apply the prettier formatting for failing tests in the diff
view so that diffs are cleaner and we don't pay for the additional
prettier calls when tests pass.

jest/customMatchers.js

@@ -74,8 +74,8 @@ function toMatchFormattedCss(received = '', argument = '') {
         )
       }
     : () => {
-        let actual = formattedReceived
-        let expected = formattedArgument
+        let actual = formatPrettier(formattedReceived)
+        let expected = formatPrettier(formattedArgument)
 
         let diffString = diff(expected, actual, {
           expand: this.expand,