Fix tests failing due to bwipeout bug

Issue: vim/vim#5431

The listener gets added twice when using `%bwipeout!`. Workaround by not
calling `bwipeout` and instead just creating new buffers.

Author: axelf4

test/test.vim

@@ -5,7 +5,6 @@ function s:TestEdits(original, EditCb, expected) abort
 	call a:EditCb()
 	silent write
 	call assert_equal(a:expected, readfile(fname))
-	%bwipeout!
 endfunction
 
 function Test_OneLineUnchanged() abort