test-metrics: Don't rely on hrtime busy loop

I've observed this test getting stuck in the 32-bit Windows CI, presumably because the value happens to be just about at the limit of i32 which is the integer type of LuaJIT when targeting x86-windows (ptrdiff_t).

This updated test is basically a port of the libuv test `metrics_idle_time_zero` in `test-metrics.c`.

A side benefit is that there's no arbitrary 1 second wait anymore.

Author: squeek502

tests/test-metrics.lua

@@ -1,29 +1,24 @@
 
 return require('lib/tap')(function (test)
 
-  test("idle time", function (print, p, expect, uv)
-    local NS_TO_MS = 1000000
-    local timeout = 1000
-    local timer = uv.new_timer()
-    local counter = 0
-    timer:start(timeout, 0, function()
-      counter = counter + 1
-      local t = uv.hrtime()
+  test("metrics_info", function (print, p, expect, uv)
+    local metrics = assert(uv.metrics_info())
+    p(metrics)
+    assert(metrics.loop_count >= 0)
+    assert(metrics.events >= 0)
+    assert(metrics.events_waiting >= 0)
+    local init_count = metrics.loop_count
 
-      -- Spin for 500 ms to spin loop time out of the delta check.
-      while uv.hrtime() - t < 600 * NS_TO_MS do end
+    local timer = uv.new_timer()
+    timer:start(0, 0, expect(function()
       timer:close()
+    end))
 
-      local metrics = uv.metrics_info()
-      p(metrics)
-      assert(metrics.loop_count > 0)
-      assert(metrics.events >= 0)
-      assert(metrics.events_waiting >= 0)
-    end)
+    uv.run()
 
-    local metrics = assert(uv.metrics_info())
+    metrics = assert(uv.metrics_info())
     p(metrics)
-    assert(metrics.loop_count >= 0)
+    assert(metrics.loop_count > init_count)
     assert(metrics.events >= 0)
     assert(metrics.events_waiting >= 0)
   end, "1.45.0")