Add kernel threading test

Kernel multithreading is working!
Threads cannot exit for now.
Scheduling is round-robin for now.

src/hulk/idt.d

@@ -225,5 +225,6 @@ public extern(C) ulong * isr(ulong vector, ulong * stack_frame)
             }
         }
     }
+    Thread.current_thread = Thread.current_thread.list_next;
     return Thread.current_thread.stack_pointer;
 }

src/hulk/test.d

@@ -7,6 +7,8 @@ import hulk.klog;
 import hulk.list;
 import hulk.time;
 import hulk.hurl.a1;
+import hulk.thread;
+import core.volatile;
 
 struct Test
 {
@@ -18,6 +20,7 @@ struct Test
         Klog.writefln("\a3Running kernel tests");
         test_pit();
         test_list();
+        test_threads();
         Klog.writefln("\a3Kernel tests complete");
     }
 
@@ -120,6 +123,44 @@ struct Test
         assert_eq(1, s3.list_count);
     }
 
+    private static __gshared ubyte thread_test_done;
+
+    private static void test_threads()
+    {
+        static void t1()
+        {
+            for (size_t i = 0; i < 5; i++)
+            {
+                Klog.writefln("thread 1");
+                Time.msleep(2000u);
+            }
+            for (;;)
+            {
+            }
+        }
+
+        static void t2()
+        {
+            Time.msleep(1000u);
+            for (size_t i = 0; i < 5; i++)
+            {
+                Klog.writefln("thread 2");
+                Time.msleep(2000u);
+            }
+            volatileStore(&thread_test_done, 1);
+            for (;;)
+            {
+            }
+        }
+
+        Thread.start(&t1);
+        Thread.start(&t2);
+
+        while (volatileLoad(&thread_test_done) == 0)
+        {
+        }
+    }
+
     private static void assert_eq(T)(T first, T second, size_t line_number = __LINE__)
     {
         if (first != second)

src/hulk/thread.d

@@ -66,6 +66,7 @@ struct Thread
         {
             threads = &this;
         }
+        list_insert_after(current_thread);
         stack_addr = A1.allocate(STACK_SIZE);
         ulong * stack_top = cast(ulong *)(stack_addr + STACK_SIZE);
         stack_pointer = stack_top - Idt.ISF_COUNT;