Add spec for Thread#native_thread_id

core/thread/native_thread_id_spec.rb

@@ -0,0 +1,31 @@
+require_relative '../../spec_helper'
+
+ruby_version_is ''...'3.1' do
+  describe "Thread#native_thread_id" do
+    it "raises NoMethodError" do
+      th = Thread.new {}
+      -> { th.native_thread_id }.should raise_error(NoMethodError)
+      th.join
+    end
+  end
+end
+
+ruby_version_is "3.1" do
+  describe "Thread#native_thread_id" do
+    platform_is_not :linux do
+      it "returns an integer" do
+        th = Thread.new {}
+        th.native_thread_id.should be_kind_of(Integer)
+        th.join
+      end
+    end
+
+    platform_is :linux do
+      it "returns nil" do
+        th = Thread.new {}
+        th.native_thread_id.should be_nil
+        th.join
+      end
+    end
+  end
+end