Test for Python project root directory

Add test files and a new test suite for detecting a Python project's
root directory.

test/test_python_find_project_root.vader

@@ -0,0 +1,11 @@
+Before:
+  call ale#test#SetDirectory('/testplugin/test')
+
+After:
+  call ale#test#RestoreDirectory()
+
+Execute(Detect root of Python project with .flake8 correctly):
+  call ale#test#SetFilename('python-test-files/python-package-project/package-name/module.py')
+  AssertEqual
+  \ ale#path#Simplify(g:dir . '/python-test-files/python-package-project'),
+  \ ale#python#FindProjectRoot(bufnr(''))