Add Tests for DIREGAPIC Enums

We are generating the new Enum classes for Compute, but we should also generate at least one test per service which ensures that these Enum classes look like we expect. For example, something like this:

```php
// in Compute/tests/Unit/V1/Enums/EnumTest.php

namespace Google\Cloud\Compute\Tests\Unit\V1\Enums;

use Google\Cloud\Compute\V1\Enums;

class EnumTest extends TestCase
{
public function testEnums()
{
$this->assertEquals('UNDEFINED_NETWORK_TIER', Enums\AccessConfig\NetworkTier::UNDEFINED_NETWORK_TIER);
// ...
// all the other enums
// ...
$this->assertEquals('UP', Enums\Zone\Status::UP);
}
}
```

This provides a sanity check that the Enum classes do not contain any syntax errors. As the existing tests are only for `InstanceClient` classes, and the enum classes do not map to `InstanceClient` classes (as far as I know), the easiest way to implement these tests is to create an `Enum` test class for all of them.