test-umount: add a simple test for swap_list_get()

The implementation seems buggy:
/* test_swap_list("/home/zbyszek/src/systemd/test/test-umount/example.swaps") */
path=0 o= f=0x0 try-ro=no dev=0:0
path=/some/swapfile2 o= f=0x0 try-ro=no dev=0:0
path=/some/swapfile o= f=0x0 try-ro=no dev=0:0
path=/dev/dm-2 o= f=0x0 try-ro=no dev=0:0

test/meson.build

@@ -177,6 +177,7 @@ test_data_files = '''
         test-umount/empty.mountinfo
         test-umount/garbled.mountinfo
         test-umount/rhbug-1554943.mountinfo
+        test-umount/example.swaps
 '''.split()
 
 if conf.get('ENABLE_RESOLVE') == 1

test/test-umount/example.swaps

@@ -0,0 +1,4 @@
+Filename				Type		Size	Used	Priority
+/dev/dm-2                               partition	8151036	2283436	-2
+/some/swapfile                          file            111     111     0
+/some/swapfile2 (deleted)               file            111     111     0