tests: remove dependencies on xglgpu.h

Replace XglGpu by xgl_testing::PhysicalGpu.  Replace MAX_GPUS by
XGL_MAX_PHYSICAL_GPUS.
diff --git a/tests/image_tests.cpp b/tests/image_tests.cpp
index 3274be0..e5673d0 100644
--- a/tests/image_tests.cpp
+++ b/tests/image_tests.cpp
@@ -76,7 +76,7 @@
 protected:
     XglDevice *m_device;
     XGL_APPLICATION_INFO app_info;
-    XGL_PHYSICAL_GPU objs[MAX_GPUS];
+    XGL_PHYSICAL_GPU objs[XGL_MAX_PHYSICAL_GPUS];
     XGL_UINT gpu_count;
     XGL_IMAGE m_image;
     XGL_GPU_MEMORY m_image_mem;
@@ -93,7 +93,7 @@
         this->app_info.apiVersion = XGL_MAKE_VERSION(0, 22, 0);
 
         err = xglInitAndEnumerateGpus(&app_info, NULL,
-                                      MAX_GPUS, &this->gpu_count, objs);
+                                      XGL_MAX_PHYSICAL_GPUS, &this->gpu_count, objs);
         ASSERT_XGL_SUCCESS(err);
         ASSERT_GE(this->gpu_count, 1) << "No GPU available";