tests/run_tests.sh: fix Meson library path being dropped

Meson automatically passes in LD_LIBRARY_PATH pointing at the correct
build directory for libfdt.so. So preserve LD_LIBRARY_PATH if it's
already set.

Signed-off-by: Brandon Maier <brandon.maier@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -55,7 +55,11 @@ fi
 if [ -z "$TEST_LIBDIR" ]; then
     TEST_LIBDIR=../libfdt
 fi
+if [ -n "$LD_LIBRARY_PATH" ]; then
+    export LD_LIBRARY_PATH="$LD_LIBRARY_PATH:$TEST_LIBDIR"
+else
     export LD_LIBRARY_PATH="$TEST_LIBDIR"
+fi
 
 export QUIET_TEST=1
 STOP_ON_FAIL=0