Suppress expected error message in fdtdump test

Since 548aea2 "fdtdump: Discourage use of fdtdump", fdtdump unconditionally
prints a message discouraging its own use except for debugging purposes.
This shows up messily in the "make check" output, so suppress it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tests/run_tests.sh

@@ -157,7 +157,7 @@ run_fdtdump_test() {
     file="$1"
     shorten_echo fdtdump-runtest.sh "$file"
     printf ":	"
-    base_run_test sh fdtdump-runtest.sh "$file"
+    base_run_test sh fdtdump-runtest.sh "$file" 2>/dev/null
 }
 
 BAD_FIXUP_TREES="bad_index \