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.

runtime/doc/builtin.txt

@@ -608,6 +608,7 @@ submatch({nr} [, {list}])	String or List
 					specific match in ":s" or substitute()
 substitute({expr}, {pat}, {sub}, {flags})
 				String	all {pat} in {expr} replaced with {sub}
+swapfilelist()			List	swap files found in 'directory'
 swapinfo({fname})		Dict	information about swap file {fname}
 swapname({buf})			String	swap file of buffer {buf}
 synID({lnum}, {col}, {trans})	Number	syntax ID at {lnum} and {col}
@@ -9314,6 +9315,17 @@ substitute({string}, {pat}, {sub}, {flags})		*substitute()*
 		Can also be used as a |method|: >
 			GetString()->substitute(pat, sub, flags)
 
+swapfilelist()						*swapfilelist()*
+		Returns a list of swap file names, like what "vim -r" shows.
+		See the |-r| command argument.  The 'directory' option is used
+		for the directories to inspect.  If you only want to get a
+		list of swap files in the current directory then temporarily
+		set 'directory' to a dot: >
+			let save_dir = &directory
+			let &directory = '.'
+			let swapfiles = swapfilelist()
+			let &directory = save_dir
+
 swapinfo({fname})					*swapinfo()*
 		The result is a dictionary, which holds information about the
 		swapfile {fname}. The available fields are:

runtime/doc/usr_41.txt

@@ -1031,6 +1031,7 @@ Buffers, windows and the argument list:
 	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