testcase for the last fix

shell/hush_test/hush-vars/var_preserved.right

@@ -0,0 +1,4 @@
+a=a
+a=a
+a=a
+OK

shell/hush_test/hush-vars/var_preserved.tests

@@ -0,0 +1,16 @@
+export a=a
+
+# external program
+a=b /bin/true
+env | grep ^a=
+
+# builtin
+a=b true
+env | grep ^a=
+
+# exec with redirection only
+# in bash, this leaks!
+a=b exec 1>&1
+env | grep ^a=
+
+echo OK