Add unit test for the new NuGet package

[OlegRa]

The backported JSON converters from the System.Text.Json have no unit test coverage right now:

• [x] Try to find the available unit tests in the System.Text.Json code base and backport them if possible
• [ ] Write new tests that demonstrate how new attributes work on different target platforms
• [ ] Think about benchmark tests for comparing built-in and backported serializers

[OlegRa]

Unit tests for JSON serialization/deserialization from BCL:

• DateOnly
• TimeOnly