Move sleep ns precision spec to kernel and omit darwin

This is really a sleep spec, so it should be tested under the
Kernel#sleep specs.

Fails consistently on MacOS (in CI) and passes consistently on
Linux and Windows so omit MacOS for now.

core/kernel/sleep_spec.rb

@@ -51,6 +51,20 @@ def o.divmod(*); [0, 0.001]; end
     t.value.should == 5
   end
 
+  platform_is_not :darwin do
+    it "sleeps with nanosecond precision" do
+      start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+      100.times do
+        sleep(0.0001)
+      end
+      end_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+
+      actual_duration = end_time - start_time
+      (actual_duration > 0.01).should == true # 100 * 0.0001 => 0.01
+      (actual_duration < 0.03).should == true
+    end
+  end
+
   ruby_version_is ""..."3.3" do
     it "raises a TypeError when passed nil" do
       -> { sleep(nil)   }.should raise_error(TypeError)

core/thread/wakeup_spec.rb

@@ -4,16 +4,4 @@
 
 describe "Thread#wakeup" do
   it_behaves_like :thread_wakeup, :wakeup
-
-  it "sleeps with nanosecond precision" do
-    start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
-    100.times do
-      sleep(0.0001)
-    end
-    end_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
-
-    actual_duration = end_time - start_time
-    (actual_duration > 0.01).should == true # 100 * 0.0001 => 0.01
-    (actual_duration < 0.03).should == true
-  end
 end