Stabilize rate limiter tests by using dependency injection

lib/req/rate_limiter/system_clock.ex

@@ -0,0 +1,13 @@
+defmodule Req.RateLimiter.SystemClock do
+  @moduledoc """
+  Real clock implementation using `System.monotonic_time/1` and `Process.sleep/1`.
+  """
+
+  @behaviour Req.RateLimiter.Clock
+
+  @impl true
+  def now, do: System.monotonic_time(:millisecond)
+
+  @impl true
+  def sleep(ms), do: Process.sleep(ms)
+end