(116548255) test decoding dynamic subclasses

[glessard]

Adapt a test from swift-corelibs-foundation, where JSON decoding could produce the wrong dynamic type. Add this test to prevent regressions. Original issue: swiftlang/swift-corelibs-foundation#4806

Addresses rdar://116548255

[glessard]

@swift-ci please test

[kperryua]

I think this extant test verifies the same behavior. Can you verify?

    func testDecodingConcreteTypeParameter() {
        let encoder = JSONEncoder()
        guard let json = try? encoder.encode(Employee.testValue) else {
            XCTFail("Unable to encode Employee.")
            return
        }

        let decoder = JSONDecoder()
        guard let decoded = try? decoder.decode(Employee.self as Person.Type, from: json) else {
            XCTFail("Failed to decode Employee as Person from JSON.")
            return
        }

        expectEqual(type(of: decoded), Employee.self, "Expected decoded value to be of type Employee; got \(type(of: decoded)) instead.")
    }

[glessard]

@kperryua you're right, that does the same thing. Thanks!
This is the test: https://github.com/apple/swift-foundation/blob/62500a5d7921cc61703c2a1b4c961adb12dc80c3/Tests/FoundationEssentialsTests/JSONEncoderTests.swift#L680