WinClosed: sort auevents.lua; improve tests

- test: reduce verbosity, condense redundancy, improve readability
- auevents.lua: keep events sorted by name. ref afd1d412fa

test/functional/helpers.lua

@@ -597,6 +597,19 @@ function module.assert_alive()
   assert(2 == module.eval('1+1'), 'crash? request failed')
 end
 
+-- Asserts that buffer is loaded and visible in the current tabpage.
+function module.assert_visible(bufnr, visible)
+  assert(type(visible) == 'boolean')
+  eq(visible, module.bufmeths.is_loaded(bufnr))
+  if visible then
+    assert(-1 ~= module.funcs.bufwinnr(bufnr),
+      'expected buffer to be visible in current tabpage: '..tostring(bufnr))
+  else
+    assert(-1 == module.funcs.bufwinnr(bufnr),
+      'expected buffer NOT visible in current tabpage: '..tostring(bufnr))
+  end
+end
+
 local function do_rmdir(path)
   local mode, errmsg, errcode = lfs.attributes(path, 'mode')
   if mode == nil then