[CoreInternsal] Make test encoder deterministic to avoid Xcode 15 flakiness

Author: ncooke3

FirebaseCore/Internal/Tests/Unit/HeartbeatTests.swift

@@ -36,14 +36,20 @@ class HeartbeatTests: XCTestCase {
   var heartbeat: Heartbeat!
   var heartbeatData: Data!
 
+  var deterministicEncoder: JSONEncoder = {
+    let encoder = JSONEncoder()
+    encoder.outputFormatting = .sortedKeys
+    return encoder
+  }()
+
   override func setUpWithError() throws {
     heartbeat = Heartbeat(
       agent: "dummy_agent",
       date: Date(timeIntervalSince1970: 1_635_739_200), // 2021-11-01
       timePeriods: [.daily],
       version: 100
     )
-    heartbeatData = try JSONEncoder().encode(heartbeat)
+    heartbeatData = try deterministicEncoder.encode(heartbeat)
   }
 
   func testHeartbeatCurrentVersion() throws {
@@ -67,7 +73,7 @@ class HeartbeatTests: XCTestCase {
     let decodedHeartbeat = try JSONDecoder()
       .decode(Heartbeat.self, from: data)
 
-    let encodedHeartbeat = try JSONEncoder()
+    let encodedHeartbeat = try deterministicEncoder
       .encode(decodedHeartbeat)
 
     // Then