patch 9.0.1010: stray warnings for existing swap files

Problem:    Stray warnings for existing swap files.
Solution:   Wipe out the buffer until it has no name and no swap file.

src/testdir/runtest.vim

@@ -364,6 +364,13 @@ func RunTheTest(test)
   call add(s:messages, message)
   let s:done += 1
 
+  " May be editing some buffer, wipe it out.  Then we may end up in another
+  " buffer, continue until we end up in an empty no-name buffer without a swap
+  " file.
+  while bufname() != '' || execute('swapname') !~ 'No swap file'
+    bwipe!
+  endwhile
+
   " Check if the test has left any swap files behind.  Delete them before
   " running tests again, they might interfere.
   let swapfiles = s:GetSwapFileList()