unit tests WP_TESTS_DIR env var necessary

Am running the unit tests and wondering if there was need for the `WP_TESTS_DIR` env var. Currently the env var is specified to be declared on the command line with every call to `phpunit` according to the [docs](http://wp-cli.org/blog/plugin-unit-tests.html)

running `phpunit` from a plugin that was scaffolded with the `wp scaffold plugin-tests` will return the following error:
```log
PHP Fatal error:  require_once(): Failed opening required '/includes/functions.php'
```
This may confuse most non-technical wordpress users. 

Suggestion:
Seeing as how the `unit-tests` are, by default, in the `blog_root/unit-tests` dir, think we should check if the env var `WP_TESTS_DIR` exists, if not then look for `ABSPATH/unit-tests` folder, if not default to throw exception.

 in [bootstrap.php](https://github.com/wp-cli/wp-cli/blob/master/templates/bootstrap.mustache)
```php
<?php
if( isset( getenv( 'WP_TESTS_DIR' ) )
    define( 'WP_TESTS_DIR', getenv( 'WP_TESTS_DIR' ) );
elseif( file_exists( ABSPATH . '/unit-tests' ) )
    define( 'WP_TESTS_DIR', ABSPATH . '/unit-tests' ) );
else
    throw new Exception( 'Please set the WP_TESTS_DIR' );

require_once WP_TESTS_DIR . '/includes/functions.php';

function _manually_load_plugin() {
	require dirname( __FILE__ ) . '/../$pluginName.php';
}
tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );

require WP_TESTS_DIR . '/includes/bootstrap.php';

```