Why isn't my hook_theme being called or rendering output?

Question

I am trying to use the hook_theme function to utilize some template files. I have the following function defined in my theme:

template.php

function my_name_theme($existing, $type, $theme, $path) {
    return array(
        'testy' => array(
            'variables' => array('title' => NULL),
            'template'  => 'testy',
        ),
    );
}

And I have the following template defined:

testy.tpl.php

<h1><?php print $title; ?></h1>

I have a content type called "Page" with a corresponding template:

node--page.tpl.php

<?php
    print 'before theme call';
    $variables = array();
    $variables['title'] = 'my title';
    print theme('testy', $variables);
    print 'after theme call';
?>

When viewing one of my "Page" content types, the rendered page only shows the following, with no title:

before theme callafter theme call

I have cleared all caches. Why isn't this working?

Answer 1

I had to add a path to my function, then it worked.

template.php

function my_name_theme($existing, $type, $theme, $path) {
    return array(
        'testy' => array(
            'variables' => array('title' => NULL),
            'template'  => 'testy',
            'path'      => drupal_get_path('theme', 'my_name') . '/templates',
        ),
    );
}