test(ssr-string.spec.js): Removed newline character, as whitespace is…

… purged in static classes

There's no need to escape newlines in static classes, as they're now replaced with a single
whitespace character

fix vuejs#12113

test/ssr/ssr-string.spec.js

@@ -1351,7 +1351,7 @@ describe('SSR: renderToString', () => {
       </div>
       `
     }, result => {
-      expect(result).toContain(`<div class="a\nb"></div>`)
+      expect(result).toContain(`<div class="a b"></div>`)
       done()
     })
   })