review: add a test that loads from a render fixture

Tests/SwiftDocCTests/Rendering/Rendering Fixtures/external-location-custom-url.json

@@ -0,0 +1,5 @@
+{
+    "identifier": "ExternalLocation.zip",
+    "url": "https://example.com/ExternalLocation.zip",
+    "type": "externalLocation"
+}

Tests/SwiftDocCTests/Rendering/SampleDownloadTests.swift

@@ -258,4 +258,28 @@ class SampleDownloadTests: XCTestCase {
 
         XCTAssertEqual(baseReference, roundTripReference)
     }
+
+    /// Ensure that an ExternalLocationReference loaded from JSON continues to encode the same
+    /// information after being decoded and re-encoded.
+    func testRoundTripExternalLocationFromFixture() throws {
+        let referenceUrl = Bundle.module.url(
+            forResource: "external-location-custom-url", withExtension: "json",
+            subdirectory: "Rendering Fixtures")!
+        let originalData = try Data(contentsOf: referenceUrl)
+
+        let encoder = JSONEncoder()
+        encoder.outputFormatting.insert(.sortedKeys)
+        let decoder = JSONDecoder()
+
+        let originalReference = try decoder.decode(ExternalLocationReference.self, from: originalData)
+        let reencodedReference = try encoder.encode(originalReference)
+
+        let roundTripReference = try decoder.decode(ExternalLocationReference.self, from: reencodedReference)
+        let finalEncodedReference = try encoder.encode(roundTripReference)
+
+        let firstJson = String(data: reencodedReference, encoding: .utf8)
+        let finalJson = String(data: finalEncodedReference, encoding: .utf8)
+
+        XCTAssertEqual(firstJson, finalJson)
+    }
 }