Relax a delta for an unstable test (ruby#14246)

https://github.com/ruby/ruby/actions/runs/16995978143/job/48186652826?pr=14244

Author: k0kubun

test/-ext-/gvl/test_last_thread.rb

@@ -15,7 +15,7 @@ def test_last_thread
       t1 = Time.now
       t = t1 - t0
 
-      assert_in_delta(1.0, t, 0.18)
+      assert_in_delta(1.0, t, 0.8)
     end;
   end
 end