hush: another testcase for "READONLY_VAR=VAL BLTIN ..."

Currently fails.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-vars/readonly0.tests

@@ -1,5 +1,5 @@
 unset a b
-
+#
 readonly a=A
 b=B
 readonly b
@@ -12,6 +12,7 @@ export -n a b
 echo Ok:$?
 env | grep -e^a= -e^b=  # shows nothing
 
+echo
 # these should all fail (despite the same value being assigned)
 # bash does not abort even in non-interactive more (in script)
 true; a=A
@@ -19,18 +20,23 @@ echo Fail:$?
 true; readonly a=A
 echo Fail:$?
 
+echo
 # in bash, assignment in export fails, but export succeeds! :)
 # we don't mimic that!
 true; export a=Z
-echo Fail:$?; true
+echo Fail:$?
 #env | grep '^a='
 #echo "^^^a is exported"
 export -n a  # undo that bashism, if it happens
 
+echo
 export b
 # this fails to both set and export a:
-a=Z env | grep '^[ab]='  # command still runs
+a=Z env | grep '^[ab]='
 echo "^^^a is not exported"
+# but external command does get executed, and $? is not mangled (stays 42):
+(exit 42); a=Z env echo Visible:$?
 
-unset a
-echo Fail:$?; true
+echo
+true; unset a
+echo Fail:$?