Don't hardcode paths when baking a plugin

Right now baking a plugin produces a bootstrap file like so:

```
<?php
/**
 * Test suite bootstrap for Example.
 */
// Customize this to be a relative path for embedded plugins.
// For standalone plugins, this should point at a CakePHP installation.
require '/var/www/cakephp.dev/config/bootstrap.php';
```

Having hardcoded paths makes things break/inflexible for distributed plugins.

This PR makes all of these "just work":

    cd my/app
    phpunit plugin/Example/tests

    cd my/app/plugin/Example
    phpunit

    git clone <some-plugin>
    cd some-plugin
    composer install
    phpunit
Couldn't we use a relative path as well?