This commit refactors `TestVim` test class in test_configuration so that
1. `test_environ_update` does not leave state (value of `powerline_config_paths`
global Vim variable) behind other tests can use.
2. `test_local_themes` does not rely on state left from `test_environ_update`,
instead using new facility for providing needed value of
`Powerline.get_config_paths` call. This facility will be used later in BAR
tests.
Ref #1256