Fix test for mutex starvation as well as small fix in thread_sync.c (ruby#15982)

Don't reset `th->running_time_us` when unlocking from `mutex_free` or
force unlocking during thread destruction. Follow-up to 994257a.

Author: luke-gruber

test/ruby/test_thread.rb

@@ -1667,22 +1667,24 @@ def test_mn_threads_sub_millisecond_sleep
 
   # [Bug #21840]
   def test_mutex_owner_doesnt_starve_waiters
-    assert_ruby_status([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
     begin;
+      require "tempfile"
+      temp = Tempfile.new("temp")
       m = Mutex.new
 
-      fib = lambda { |n|
+      def fib(n)
         return n if n <= 1
         fib(n - 1) + fib(n - 2)
-      }
+      end
 
       t1_running = false
-      t1 = Thread.new do
+      Thread.new do
         t1_running = true
         loop do
           fib(20)
           m.synchronize do
-            File.open(__FILE__) { } # reset timeslice due to blocking operation
+            File.open(temp.path) { } # reset timeslice due to blocking operation
           end
         end
       end

thread_sync.c

@@ -466,7 +466,7 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th, rb_serial_t ec_serial)
     struct sync_waiter *cur = 0, *next;
 
 
-    if (mutex->wait_waking) {
+    if (mutex->wait_waking && ec_serial) {
         uint32_t saved = mutex->saved_running_time_us;
         if (th->running_time_us < saved) {
             th->running_time_us = saved;