bug #31040 [BrowserKit] Fixed BC-break introduced by rename of Client…

… to Browser (Devristo)

This PR was squashed before being merged into the 4.3-dev branch (closes #31040).

Discussion
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[BrowserKit] Fixed BC-break introduced by rename of Client to Browser

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #31039
| License       | MIT
| Doc PR        |

Since #30541 the inheritance hierarchy of `\Symfony\Component\BrowserKit\Client` has changed. Notably the test.client no longer is an instance of `\Symfony\Component\BrowserKit\Client`.

This PR uses `class_alias` to fix the class hierarchy similarly as has been done in Twig. In this case I copied the approach of `Twig_TokenParser_AutoEscape` and `\Twig\TokenParser\AutoEscapeTokenParser`

Commits
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6a94dea [BrowserKit] Fixed BC-break introduced by rename of Client to Browser