Add test for field ordering

[levinson]

Pull Request Checklist

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• Have you added tests for any changed functionality? N/A

Purpose

This PR demonstrates #1038. The new test fails due to serialized field order when compiled with Scala 3:

[info] - should preserve field ordering *** FAILED *** (33 milliseconds)
[info]   "{"[a":42,"m1":"foo","b":"baz","c":"bar","z":1.2]}" did not equal "{"[m1":"foo","c":"bar","z":1.2,"a":42,"b":"baz"]}" (JsonSharedSpec.scala:454)

[cchantep]

Suggested change

	serialized.mustEqual("""{"m1":"foo","c":"bar","z":1.2,"a":42,"b":"baz"}""")
	serialized.mustEqual("""{"m1":"foo","c":"bar","z":1.2,"a":42,"b":"baz"}""")

[jonaskoelker-jypo]

Would it make sense to test the field ordering at the JsObject level in addition to the String level?

I note the comment on the fields method on JsObject: it gives the fields in order. Based on this, I would—naively?—expect the Seq returned by fields to be in declaration order when using the Json.writes macro and friends. It would be a real nicety to have a test for that as well.

play-json/play-json/shared/src/main/scala/play/api/libs/json/JsValue.scala

Lines 116 to 123 in 85a8aa0

	case class JsObject(
	private[json] val underlying: Map[String, JsValue]
	) extends JsValue {
	/**
	* The fields of this JsObject in the order passed to the constructor
	*/
	def fields: collection.Seq[(String, JsValue)] = underlying.toSeq

[On the other hand: I can argue that a JsObject doesn't contain any information about the field declaration order except the ordering of the ImmutableLinkedHashMap in underlying. Ergo: if it's wrong at the JsObject level there's no way Json.stringify can correct it, and thus this test case implicitly also verifies that underlying is ordered correctly. That feels a bit indirect for my tastes—I lean towards an independent test of that—but I figured it deserved to be mentioned.]