use value of DIRECTORY_SEPARATOR to detect Windows

This commit unifies the detection of Windows builds across the Symfony
codebase.
[Form] Remove a redundant test.

This is exactly the same as testValidateTokenOnSubmitIfRootAndCompoundUsesTypeClassAsIntentionIfEmptyFormName().
minor #13172 [Form] Remove a redundant test (jakzal)

This PR was merged into the 2.3 branch.

Discussion
----------

[Form] Remove a redundant test

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

As a result of changing `bind()` to `submit()` in #13161, the `testValidateTokenOnBindIfRootAndCompoundUsesTypeClassAsIntentionIfEmptyFormName()` is exactly the same as `testValidateTokenOnSubmitIfRootAndCompoundUsesTypeClassAsIntentionIfEmptyFormName()`. This makes the form tests to fail on 2.5 branch, as after a merge csrf provider is used instead of the token manager.

Commits
-------

ae10615 [Form] Remove a redundant test.
minor #13147 [2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[2.3] for consistency, use value of DIRECTORY_SEPARATOR to detect Windows

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

This commit unifies the detection of Windows builds across the Symfony
codebase.

Commits
-------

20a427d use value of DIRECTORY_SEPARATOR to detect Windows