Add availability guards to Duration conversion tests. (#1746)

For some reason the compiler didn't complain at me for not having these,
despite it complaining at me (correctly) if they weren't in the runtime.
My tests ran locally fine without the guards, but our Bazel builds were
failing without them.

Tests/SwiftProtobufTests/Test_Duration.swift

@@ -314,6 +314,10 @@ final class Test_Duration: XCTestCase, PBTestHelpers {
     }
 
     func testConvertFromStdlibDuration() throws {
+        guard #available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) else {
+            throw XCTSkip("Duration is not supported on this platform")
+        }
+
         // Full precision
         do {
             let sd = Duration.seconds(123) + .nanoseconds(123_456_789)
@@ -372,6 +376,10 @@ final class Test_Duration: XCTestCase, PBTestHelpers {
     }
 
     func testConvertToStdlibDuration() throws {
+        guard #available(macOS 13.0, iOS 16.0, watchOS 9.0, tvOS 16.0, *) else {
+            throw XCTSkip("Duration is not supported on this platform")
+        }
+
         do {
             let pd = Google_Protobuf_Duration(seconds: 123, nanos: 123_456_789)
             let sd = Duration(pd)