tests/aiger: Fix pipe hiding diff exit status

tests/aiger/run-test.sh

@@ -62,5 +62,8 @@ done
 # make gold with: rm gold/*; yosys --no-version -p "test_cell -aigmap -w gold/ -n 1 -s 1 all"
 rm -rf gate; mkdir gate
 ../../yosys --no-version -p "test_cell -aigmap -w gate/ -n 1 -s 1 all"
-diff --brief gold gate | tee aigmap.err
+(
+    set -o pipefail
+    diff --brief gold gate | tee aigmap.err
+)
 rm aigmap.err