tests: add functional test for BufModifiedSet

test/functional/autocmd/bufmodifiedset_spec.lua

@@ -0,0 +1,20 @@
+local helpers = require('test.functional.helpers')(after_each)
+
+local clear = helpers.clear
+local eq = helpers.eq
+local eval = helpers.eval
+local source = helpers.source
+
+describe('BufModified', function()
+  before_each(clear)
+
+  it('is triggered when modified and un-modified', function()
+    source([[
+    let g:modified = 0
+    autocmd BufModifiedSet * let g:modified += 1
+    execute "normal! aa\<Esc>"
+    execute "normal! u"
+    ]])
+    eq(2, eval('g:modified'))
+  end)
+end)