RegexTester: fix getting reference to regex field

RegexTester: fix getting reference to regex field

the regular expression field is not necessarily after the regex tester
(mostly it is before). In order to be more robust, use the child
method of the parent window instead of previousSibling of the button.

The child method gets any direct descendant matching the query, the
sibling one is always directional and there's no method for "any
sibling".

Tested with:
* mailproxy whitelist
* match field
* match filename
* who object

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>