When testing Vim do not rely on test order and do not save state

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

tests/lib/config_mock.py

@@ -182,10 +182,17 @@ def select_renderer(simpler_renderer=False):
 	renderer = EvenSimplerRenderer if simpler_renderer else SimpleRenderer
 
 
-def get_powerline_raw(helpers, PowerlineClass, **kwargs):
+def get_powerline_raw(helpers, PowerlineClass, replace_gcp=False, **kwargs):
 	if not isinstance(helpers, TestHelpers):
 		helpers = TestHelpers(helpers)
 	select_renderer(kwargs.pop('simpler_renderer', False))
+
+	if replace_gcp:
+		class PowerlineClass(PowerlineClass):
+			@staticmethod
+			def get_config_paths():
+				return ['/']
+
 	pl = PowerlineClass(
 		config_loader=TestConfigLoader(
 			_helpers=helpers,

tests/test_configuration.py

@@ -750,10 +750,10 @@ class TestVim(TestCase):
 	def test_environ_update(self):
 		# Regression test: test that segment obtains environment from vim, not 
 		# from os.environ.
+		import tests.vim as vim_module
+		with vim_module._with('globals', powerline_config_paths=['/']):
 			from powerline.vim import VimPowerline
 			import powerline as powerline_module
-		import vim
-		vim.vars['powerline_config_paths'] = ['/']
 			with swap_attributes(config, powerline_module):
 				with vim_module._with('environ', TEST='abc'):
 					with get_powerline_raw(config, VimPowerline) as powerline:
@@ -769,7 +769,7 @@ class TestVim(TestCase):
 		from powerline.vim import VimPowerline
 		import powerline as powerline_module
 		with swap_attributes(config, powerline_module):
-			with get_powerline_raw(config, VimPowerline) as powerline:
+			with get_powerline_raw(config, VimPowerline, replace_gcp=True) as powerline:
 				powerline.add_local_theme('tests.matchers.always_true', {
 					'segment_data': {
 						'foo': {