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"}""")