tests/throttle: Clean up global variable shadowing

Follow all other tests pattern from this file, use the
global 'cfg' variable to fix:

  tests/unit/test-throttle.c:621:20: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
      ThrottleConfig cfg;
                     ^
  tests/unit/test-throttle.c:28:23: note: previous declaration is here
  static ThrottleConfig cfg;
                        ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Acked-by: Alberto Garcia <berto@igalia.com>
Message-ID: <20231009100251.56019-3-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>