patch 9.0.1007: there is no way to get a list of swap file names

Problem:    There is no way to get a list of swap file names.
Solution:   Add the swapfilelist() function.  Use it in the test script to
            clean up.  Remove deleting individual swap files.
diff --git a/src/testdir/test_swap.vim b/src/testdir/test_swap.vim
index 5237edc..c776ca5 100644
--- a/src/testdir/test_swap.vim
+++ b/src/testdir/test_swap.vim
@@ -110,6 +110,15 @@
   w
   let fname = s:swapname()
   call assert_match('Xswapinfo', fname)
+
+  let nr = 0
+  for name in swapfilelist()
+    if name =~ '[\\/]' .. fname .. '$'
+      let nr += 1
+    endif
+  endfor
+  call assert_equal(1, nr)
+
   let info = fname->swapinfo()
 
   let ver = printf('VIM %d.%d', v:version / 100, v:version % 100)