add a trait for helping with expectations.

Permit this:

    $Foo
        ->expects($this->next())
        ->method(...
    $Foo
        ->expects($this->next())
        ->method(...
    $Bar
        ->expects($this->next('Bar'))
        ->method(...
    $Bar
        ->expects($this->next('Bar'))
        ->method(...
    $Foo
        ->expects($this->next())
        ->method(...