patch 8.2.1384: no ATTENTION prompt for :vimgrep first match file

Problem:    No ATTENTION prompt for :vimgrep first match file.
Solution:   When there is an existing swap file do not keep the dummy buffer.
            (closes #6649)
diff --git a/src/testdir/runtest.vim b/src/testdir/runtest.vim
index f3609f8..f431748 100644
--- a/src/testdir/runtest.vim
+++ b/src/testdir/runtest.vim
@@ -91,6 +91,9 @@
 let s:test_script_fname = expand('%')
 au! SwapExists * call HandleSwapExists()
 func HandleSwapExists()
+  if exists('g:ignoreSwapExists')
+    return
+  endif
   " Ignore finding a swap file for the test script (the user might be
   " editing it and do ":make test_name") and the output file.
   " Report finding another swap file and chose 'q' to avoid getting stuck.