add more seq tests

test/test_python.sh

@@ -25,3 +25,7 @@ Darwin)
 esac
 
 $PYTHON -m unittest -v test_all 
+
+echo rerunning with VIPS_STALL enabled ...
+export VIPS_STALL=1
+$PYTHON -m unittest -v test_all 

test/test_thumbnail.sh

@@ -24,6 +24,9 @@ break_threshold() {
 	return $(echo "$diff > $threshold" | bc -l)
 }
 
+# run the test with VIPS_STALL enabled to stress the seq system
+export VIPS_STALL=1
+
 size=1000
 while [ $size -gt 99 ]; do
 	printf "testing size to $size ... "