vim-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.

c216a7a21a

vim-patch:9.0.1005: a failed test may leave a swap file behind

Problem:    A failed test may leave a swap file behind.
Solution:   Delete the swap file to avoid another test to fail.  Use another
            file name.

d0f8d39d20

Cherry-pick test_window_cmd.vim changes from patch 8.2.1593.
Remove FUNC_ATTR_UNUSED from eval functions as fptr is always unused.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.lua

@@ -400,6 +400,7 @@ return {
     strwidth={args=1, base=1, fast=true},
     submatch={args={1, 2}, base=1},
     substitute={args=4, base=1},
+    swapfilelist={},
     swapinfo={args=1, base=1},
     swapname={args=1, base=1},
     synID={args=3},