Remove incorrect casts from tests/string_list.at

The list_order argument already has the correct pointer type const
char * const * (after array-to-pointer decay).

Submitted upstream: <https://github.com/abrt/libreport/pull/802>

diff --git a/tests/string_list.at b/tests/string_list.at
index 6af162a2f8da3919..2118298da4de0ec3 100644
--- a/tests/string_list.at
+++ b/tests/string_list.at
@@ -22,13 +22,13 @@ int main(void)
             NULL
     };
 
-    int index = libreport_index_of_string_in_list(FILENAME_REASON, (char**) list_order);
+    int index = libreport_index_of_string_in_list(FILENAME_REASON, list_order);
     assert(index == 0);
 
-    index = libreport_index_of_string_in_list(FILENAME_COUNT, (char**) list_order);
+    index = libreport_index_of_string_in_list(FILENAME_COUNT, list_order);
     assert(index == 5);
 
-    index = libreport_index_of_string_in_list("other", (char**) list_order);
+    index = libreport_index_of_string_in_list("other", list_order);
     assert(index < 0);
 
     return 0;