added ExpressionVoterTest
minor #9696 [Security] added test for ExpressionVoter.php (liuggio)

This PR was submitted for the 2.4-dev branch but it was merged into the 2.4 branch instead (closes #9696).

Discussion
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[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.

Commits
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e9de00f added ExpressionVoterTest