Address minor @todo in StreamFilterTest after next release of encrypt module

\Drupal\Tests\file_encrypt\Unit\StreamFilterTest::setUp() includes the following @todo, which should be addressed when possible:

// @todo The current alpha release of encrypt module incorrectly depends on
//   a concretion rather than an interface, so we must mock accordingly.
//   Use \Drupal\encrypt\EncryptionProfileInterface once the next release
//   comes out.
$this->encryptionProfile = $this->getMockBuilder(EncryptionProfile::class)
  ->disableOriginalConstructor()
  ->getMock();