Add a test case for custom snippet for `enable_completed_snippet`

test/functions.vim

@@ -128,5 +128,10 @@ function! s:suite.get_completed_snippet() abort
         \ 'word' : 'for[', 'abbr' : '',
         \ 'menu' : '', 'info' : ''
         \ }, ''), '${1}]${2}')
+
+  call s:assert.equals(neosnippet#parser#_get_completed_snippet({
+        \ 'word' : 'something', 'abbr' : 'something(else)',
+        \ 'menu' : '', 'info' : '', 'snippet' : '(${1:custom})${2}'
+        \ }, ''), '(${1:custom})${2}')
 endfunction