Skip testing mipmaps if extension is not available

'./test_image_streams test_mipmaps CL_FILTER_NEAREST' already
skips many tests, but doesn't do so for cl_ext_image_raw10_raw12,
causing it to fail.
Add a check similar to how it is done in the other tests already.

test_conformance/images/kernel_read_write/test_loops.cpp

@@ -175,6 +175,18 @@ int test_read_image_formats(cl_device_id device, cl_context context,
     bool flipFlop[2] = { false, true };
     int normalizedIdx, floatCoordIdx;
 
+    if (gTestMipmaps)
+    {
+        if (0 == is_extension_available(device, "cl_khr_mipmap_image"))
+        {
+            log_info("-----------------------------------------------------\n");
+            log_info("This device does not support "
+                     "cl_khr_mipmap_image.\nSkipping mipmapped image test. \n");
+            log_info(
+                "-----------------------------------------------------\n\n");
+            return 0;
+        }
+    }
 
     // Use this run if we were told to only run a certain filter mode
     if (gFilterModeToUse != (cl_filter_mode)-1