[Security] Use hash_equals for constant-time string comparison
[Security] Add more tests for StringUtils::equals
minor #11822 [Security] Use hash_equals for constant-time string comparison (again) (dunglas)

This PR was merged into the 2.3 branch.

Discussion
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[Security] Use hash_equals for constant-time string comparison (again)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

Use the `hash_equals` function (introduced in PHP 5.6) for timing attack safe string comparison when available.

Add in the DocBlock that length will leak (https://github.com/symfony/symfony/pull/11797#issuecomment-53990712).

Commits
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3071557 [Security] Add more tests for StringUtils::equals
03bd74b [Security] Use hash_equals for constant-time string comparison