[add] winwidth's test

autoload/airline/util.vim

@@ -16,7 +16,7 @@ let s:focusgained_ignore_time = 0
 " TODO: Try to cache winwidth(0) function
 " e.g. store winwidth per window and access that, only update it, if the size
 " actually changed.
-function! airline#util#winwidth(...)
+function! airline#util#winwidth(...) abort
   let nr = get(a:000, 0, 0)
   if get(g:, 'airline_statusline_ontop', 0)
     return &columns

t/util.vim

@@ -20,6 +20,20 @@ describe 'util'
     Expect airline#util#append('1', 0) == '   1'
   end
 
+  it 'should be same &columns'
+    let g:airline_statusline_ontop = 1
+    Expect airline#util#winwidth() == &columns
+  end
+
+  it 'should be same winwidth(0)'
+    let g:airline_statusline_ontop = 0
+    Expect airline#util#winwidth() == winwidth(0)
+  end
+
+  it 'should be same winwidth(30)'
+    Expect airline#util#winwidth(30, 0) == winwidth(30)
+  end
+
   it 'has prepend wrapper function'
     Expect airline#util#prepend('', 0) == ''
     Expect airline#util#prepend('1', 0) == '1  '