Remove error highlights when buffers are cleaned up

test/test_highlight_placement.vader

@@ -74,3 +74,20 @@ Execute(Existing highlights should be kept):
   \   {'group': 'ALEWarning', 'id': 8, 'priority': 10, 'pos1': [4, 1, 1]},
   \ ],
   \ getmatches()
+
+" This test is important for preventing ALE from showing highlights for
+" the wrong files.
+Execute(Highlights set by ALE should be removed when buffer cleanup is done):
+  call ale#engine#InitBufferInfo(bufnr('%'))
+
+  call ale#highlight#SetHighlights(bufnr('%'), [
+  \ {'bufnr': bufnr('%'), 'type': 'E', 'lnum': 3, 'col': 2},
+  \])
+
+  AssertEqual
+  \ [{'group': 'ALEError', 'id': 9, 'priority': 10, 'pos1': [3, 2, 1]}],
+  \ getmatches()
+
+  call ale#cleanup#Buffer(bufnr('%'))
+
+  AssertEqual [], getmatches()