Configuration – System Config Tab Not Showing in CE 1.8

I'm trying to add a simple tab to the system configuration in Magento CE 1.8.0.0. It's not showing at all with my code, as follows:

File: app/code/community/Test/Module/etc/adminhtml.xml

<?xml version="1.0"?>
<config>
    <acl>
        <resources>
            <all>
                <title>Allow Everything</title>
            </all>
            <admin>
                <children>
                    <system>
                        <children>
                            <config>
                                <children>
                                    <test_module translate="title" module="test_module">
                                        <title>Test Tabs Section</title>
                                        <sort_order>999</sort_order>
                                    </test_module>
                                </children>
                            </config>
                        </children>
                    </system>
                </children>
            </admin>
        </resources>
    </acl>
</config>

File: app/code/community/Test/Module/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Test_Module>
            <version>0.1.0</version>
        </Test_Module>
    </modules>
    <global>
        <helpers>
            <test_module>
                <class>Test_Module_Helper</class>
            </test_module>
        </helpers>
    </global>
</config>

File: app/code/community/Test/Module/etc/system.xml

<?xml version="1.0"?>
<config>
    <tabs>
        <test_module translate="label" module="test_module">
            <label>Test Module</label>
            <sort_order>999</sort_order>
        </test_module>
    </tabs>
</config>

File: app/code/community/Test/Module/Helper/Data.php

<?php
class Test_Module_Helper_Data extends Mage_Core_Helper_Data
{}

File: app/etc/modules/Test_Module.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Test_Module>
            <active>true</active>
            <codePool>community</codePool>
            <depends>
                <Mage_Adminhtml />
            </depends>
        </Test_Module>
    </modules>
</config>

I'm expecting an empty tab in the admin's system configuration section, but nothing shows at all. I have emptied the cache and re-logged several times. What am I doing wrong? Thanks!