Simplify client instance and loginAs

Currently, if we want to use an another user for auth, we have to do this:

```php
public function testPaymentsAreNotShownOnSearch()
{
$this->loginAs($this->getReference('user_super_admin'));
$client = $this->makeClient();
}
```

Why not make `makeClient` accepting `UserInterface` instances? And then we can simplify the call like this:

```php
public function testPaymentsAreNotShownOnSearch()
{
$client = $this->makeClient($this->getReference('user_super_admin'));
}
```

And same for `fetchContent`.

What do you think?

Note: We may also need #345.

Start by locating the existing makeClient, fetchContent, and loginAs helpers and review how #345 affects them. Update the helpers to accept UserInterface instances so authentication can be supplied directly, then verify the functional test usage no longer requires a separate loginAs call.

Written by the indexing model from the issue text.