[9.x] Fix ExcludeIf regression to use Closure over is_callable()

[BrandonSurowiec]

This fixes a regression introduced by #41931. In the original test, the string phpinfo was being pass into the constructor. After the rewrite, this check was changed to foobar.

// Original Test
$this->expectException(InvalidArgumentException::class);

$rule = new ExcludeIf('phpinfo');

Why this matters is that is_callable('phpinfo') will evaluate to TRUE as it is a function in the global namespace. This switches the check from is_callable($condition) to the more strict $condition instanceof Closure and updates the test to check the string phpinfo once again.