[2.3] [HttpFoundation] Fixed the testStarted method name

| 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

##### This pull request fixes the testStarted method name.

The incorrect method name was introduced by https://github.com/symfony/symfony/commit/b9d3c92ca9a2c02373f014deea03aee9d278e692#diff-b3a49132e88c1d218317fd4bf46e389dL211.