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.

## Examples

This always worked, and still does (because it doesn't use the plugin's test bootstrap file):

    cd my/app
    phpunit plugin/Example/tests

This currently only works on the machine that baked the plugin (or more accurately on any machine with the same dir structure as the machine that baked the plugin), and still does:

    cd my/app/plugin/Example
    phpunit

This one currently doesn't work, but will with this PR:

    git clone <some-plugin>
    cd some-plugin
    composer install
    phpunit

The last example is possibly most interesting for travis use.

Though in addition to that I'm in the habit of downloading, composer install-ing, and phpunit-ing anything I'm considering using to see if it works - and it's disheartening that by default 3.0 plugins fail this smoke test.