[Security] added test for ExpressionVoter.php

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none


Added the missing test for `ExpressioneVoter`,
Fixed small docblock.