fate: use 'run' helper for seek-test

This is simpler, and the actual seek-test command is printed with V=1.
Signed-off-by: Mans Rullgard <mans@mansr.com>

tests/fate-run.sh

@@ -104,7 +104,7 @@ seektest(){
                  file=$(echo tests/data/$d/$file)
                  ;;
     esac
-    $target_exec $target_path/libavformat/seek-test $target_path/$file
+    run libavformat/seek-test $target_path/$file
 }
 
 mkdir -p "$outdir"