add test for entering a popup window

test/popup.vim

@@ -0,0 +1,19 @@
+if !exists('*popup_menu') || !exists('*win_execute')
+  finish
+endif
+
+let s:suite = themis#suite('popup')
+let s:assert = themis#helper('assert')
+
+function! s:suite.before_each()
+  let g:lightline = {}
+  call lightline#init()
+  tabnew
+  tabonly
+endfunction
+
+function! s:suite.win_execute_setfiletype()
+  let id = popup_menu(['aaa', 'bbb'], {})
+  call win_execute(id, 'setfiletype vim')
+  call popup_close(id)
+endfunction