TOOLS/command-test.lua: add test for async subprocess command blocking

On master, this takes around 4000ms for me. While with #14973
it completes in 10ms

Author: llyyr

TOOLS/lua/command-test.lua

@@ -107,6 +107,16 @@ mp.observe_property("vo-configured", "bool", function(_, v)
     -- Runs detached; should be killed on player exit (forces timeout)
     mp.command_native({_flags={"async"}, name = "subprocess",
                        playback_only = false, args = {"sleep", "inf"}})
+
+    -- Rapidly dispatching many subprocesses asynchronously must not block the
+    -- main thread. If it takes multiple seconds we're blocking the main thread.
+    local rapid_start = mp.get_time()
+    for i = 1, 1000 do
+        mp.command_native_async({name = "subprocess", args = {"true"}},
+            function() end)
+    end
+    print(string.format("done rapid subprocess dispatch in %.3fms",
+        (mp.get_time() - rapid_start) * 1000))
 end)
 
 local counter