hush: add readonly testcase, fix fallout

function                                             old     new   delta
helper_export_local                                  185     214     +29
run_pipe                                            1549    1560     +11
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 40/0)               Total: 40 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/shell/hush_test/hush-vars/readonly0.right b/shell/hush_test/hush-vars/readonly0.right
new file mode 100644
index 0000000..9688d2e
--- /dev/null
+++ b/shell/hush_test/hush-vars/readonly0.right
@@ -0,0 +1,12 @@
+readonly a=A
+readonly b=B
+Ok:0
+hush: a=A: readonly variable
+Fail:1
+hush: a=A: readonly variable
+Fail:1
+hush: a=A: readonly variable
+Fail:1
+Visible:0
+hush: a: readonly variable
+Fail:1