Fixes unit tests

I have found a problem when running the unit tests local, first I have tried:

```
$ ./bin/zephir build
$ phpunit

PHPUnit 3.7.22 by Sebastian Bergmann.

Configuration read from /home/rodrigomoyle/workspace/zephir/phpunit.xml

PHP Fatal error:  Class 'Test\Arithmetic' not found in /home/rodrigomoyle/workspace/zephir/unit-tests/Extension/ArithmeticTest.php on line 28
...
```

And tried to load the extension dynamically:

```
$ php -d="extension=ext/modules/test.so" `which phpunit`

PHPUnit 3.7.22 by Sebastian Bergmann.

Configuration read from /home/rodrigomoyle/workspace/zephir/phpunit.xml

................................................PHP Fatal error:  Class 'Test\ExitDie' not found in /home/rodrigomoyle/workspace/zephir/unit-tests/Extension/fixtures/exit.php on line 3
PHP Stack trace:
PHP   1. {main}() /home/rodrigomoyle/workspace/zephir/unit-tests/Extension/fixtures/exit.php:0
PHP Fatal error:  Class 'Test\ExitDie' not found in /home/rodrigomoyle/workspace/zephir/unit-tests/Extension/fixtures/exit_string.php on line 5
PHP Stack trace:
PHP   1. {main}() /home/rodrigomoyle/workspace/zephir/unit-tests/Extension/fixtures/exit_string.php:0
F..............  63 / 153 ( 41%)
```

Now, its easy to run the unit tests locally:

```
$ make test
```

And works!
Could you please remove this hardcoded path to the extension?