Add a has() wrapper we can override for tests, and add a function for generating paths up to the root directory to search through

test/test_path_upwards.vader

@@ -0,0 +1,46 @@
+After:
+  let g:ale_has_override = {}
+
+Execute(ale#path#Upwards should return the correct path components for Unix):
+  " Absolute paths should include / on the end.
+  AssertEqual
+  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz')
+  AssertEqual
+  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz///')
+  " Relative paths do not.
+  AssertEqual
+  \ ['foo/bar/baz', 'foo/bar', 'foo'],
+  \ ale#path#Upwards('foo/bar/baz')
+  AssertEqual
+  \ ['foo2/bar', 'foo2'],
+  \ ale#path#Upwards('foo//..////foo2////bar')
+  " Expect an empty List for empty strings.
+  AssertEqual [], ale#path#Upwards('')
+
+Execute(ale#path#Upwards should return the correct path components for Windows):
+  let g:ale_has_override = {'win32': 1}
+
+  AssertEqual
+  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+  \ ale#path#Upwards('C:\foo\bar\baz')
+  AssertEqual
+  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+  \ ale#path#Upwards('C:\foo\bar\baz\\\')
+  AssertEqual
+  \ ['/foo\bar\baz', '/foo\bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz')
+  AssertEqual
+  \ ['foo\bar\baz', 'foo\bar', 'foo'],
+  \ ale#path#Upwards('foo/bar/baz')
+  AssertEqual
+  \ ['foo\bar\baz', 'foo\bar', 'foo'],
+  \ ale#path#Upwards('foo\bar\baz')
+  " simplify() is used internally, and should sort out \ paths when actually
+  " running Windows, which we can't test here.
+  AssertEqual
+  \ ['foo2\bar', 'foo2'],
+  \ ale#path#Upwards('foo//..///foo2////bar')
+  " Expect an empty List for empty strings.
+  AssertEqual [], ale#path#Upwards('')