t/stress: add a test that usually triggers a bug using 950-curse.

It looks like we're updating the stamp for t/countall while another
task is replacing the file, which suggests a race condition in our
state management database.

t/stress.do

@@ -0,0 +1,10 @@
+rm -f $1.log
+exec >$1.log
+
+# This test twiddles the same files over and over, and seems to trigger race conditions
+# in redo if run repeatedly with a large redo -j.
+for d in $(seq 25); do
+    echo "stress test: cycle $d" >&2
+    ./flush-cache 2>&1
+    redo 950-curse/all 2>&1 || { rv=$?; echo "stress test: log is $1.log" >&2; exit $rv; }
+done