Add a function for getting matches, and use it to simplify a lot of code

test/handler/test_rubocop_handler.vader

@@ -4,28 +4,24 @@ Execute(The rubocop handler should parse lines correctly):
   AssertEqual
   \ [
   \   {
-  \     'bufnr': 347,
   \     'lnum': 83,
   \     'col': 29,
   \     'text': 'Prefer single-quoted strings...',
   \     'type': 'W',
   \   },
   \   {
-  \     'bufnr': 347,
   \     'lnum': 12,
   \     'col': 2,
   \     'text': 'Some error',
   \     'type': 'E',
   \   },
   \   {
-  \     'bufnr': 347,
   \     'lnum': 10,
   \     'col': 5,
   \     'text': 'Regular warning',
   \     'type': 'W',
   \   },
   \   {
-  \     'bufnr': 347,
   \     'lnum': 11,
   \     'col': 1,
   \     'text': 'Another error',