Fix flaky tests (#1229) (#1231)

(cherry picked from commit 84b6377)

Co-authored-by: Matthias Kurz <m.kurz@irregular.at>

Author: mergify[bot]

play-json/jvm/src/test/scala/play/api/libs/json/JsonSpec.scala

@@ -15,6 +15,9 @@ import play.api.libs.json.Json._
 import play.api.libs.json.jackson.JacksonJson
 
 class JsonSpec extends org.specs2.mutable.Specification {
+
+  sequential
+
   "JSON".title
 
   import java.text.SimpleDateFormat