dafny: update filename handling

cf. #3475, #3390

ale_linters/dafny/dafny.vim

@@ -16,7 +16,7 @@ function! ale_linters#dafny#dafny#Handle(buffer, lines) abort
 
     for l:match in ale#util#GetMatches(a:lines, '\v(.*)\((\d+),(\d+)\): (Verification of .{-} timed out after \d+ seconds)')
         call add(l:output, {
-        \     'bufnr': a:buffer,
+        \     'filename': l:match[1],
         \     'col': l:match[3] + 0,
         \     'lnum': l:match[2] + 0,
         \     'text': l:match[4],

test/handler/test_dafny_handler.vader

@@ -22,7 +22,7 @@ Execute(The Dafny handler should parse output correctly):
   \     'type': 'W'
   \   },
   \   {
-  \     'bufnr': 0,
+  \     'filename': 'File.dfy',
   \     'col': 45,
   \     'lnum': 123,
   \     'text': "Verification of 'Impl$$_22_Proof.__default.PutKeepsMapsFull' timed out after 2 seconds",