add RUN_EXPENSIVE_TESTS junk

tests/rm/hash

@@ -3,6 +3,18 @@
 # Before then, rm would fail occasionally, sometimes via
 # a failed assertion, others with a seg fault.
 
+if test "$RUN_EXPENSIVE_TESTS" != yes; then
+  cat <<EOF >&2
+$0: This test is relatively expensive, so it is disabled by default.
+To run it anyway, rerun make check with the RUN_EXPENSIVE_TESTS
+environment variable set to yes.  E.g.,
+
+  env RUN_EXPENSIVE_TESTS=yes make check
+
+EOF
+  exit 77
+fi
+
 if test "$VERBOSE" = yes; then
   set -x
   rm --version