test/*.sh : Fix MALLOC_PERTURB initialization.

Was using '$(date +%N)', but that doesn't work on OSX or *BSD. Switched
to using '$(date +%s)'.
diff --git a/test/test_metaflac.sh b/test/test_metaflac.sh
index 86970a6..c6f7ed1 100755
--- a/test/test_metaflac.sh
+++ b/test/test_metaflac.sh
@@ -48,7 +48,7 @@
 LD_LIBRARY_PATH=`pwd`/../objs/$BUILD/lib:$LD_LIBRARY_PATH
 export LD_LIBRARY_PATH
 export MALLOC_CHECK_=3
-export MALLOC_PERTURB_=$((1$(date +%N) % 255 + 1))
+export MALLOC_PERTURB_=$(($(date +%s) % 255 + 1))
 PATH=`pwd`/../src/flac:$PATH
 PATH=`pwd`/../src/metaflac:$PATH
 PATH=`pwd`/../objs/$BUILD/bin:$PATH