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/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 43a13d9..850a4c5 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -1031,6 +1031,7 @@
 	getwininfo()		get a list with window information
 	getchangelist()		get a list of change list entries
 	getjumplist()		get a list of jump list entries
+	swapfilelist()		list of existing swap files in 'directory'
 	swapinfo()		information about a swap file
 	swapname()		get the swap file path of a buffer