patch 8.1.2225: the "last used" info of a buffer is under used

Problem:    The "last used" info of a buffer is under used.
Solution:   Add "lastused" to getbufinfo(). List buffers sorted by last-used
            field. (Andi Massimino, closes #4722)
diff --git a/src/testdir/test_excmd.vim b/src/testdir/test_excmd.vim
index 509d78d..992fc3d 100644
--- a/src/testdir/test_excmd.vim
+++ b/src/testdir/test_excmd.vim
@@ -19,3 +19,28 @@
   normal vv
   call assert_fails(":'<,'>echo 1", 'E481:')
 endfunc
+
+func Test_buffers_lastused()
+  call test_settime(localtime() - 2000) " middle
+  edit bufa
+  enew
+  call test_settime(localtime() - 10)   " newest
+  edit bufb
+  enew
+  call test_settime(1550010000)	        " oldest
+  edit bufc
+  enew
+  call test_settime(0)
+  enew
+
+  let ls = split(execute('buffers t', 'silent!'), '\n')
+  let bufs = ls->map({i,v->split(v, '"\s*')[1:2]})
+  call assert_equal(['bufb', 'bufa', 'bufc'], bufs[1:]->map({i,v->v[0]}))
+  call assert_match('1[0-3] seconds ago', bufs[1][1])
+  call assert_match('\d\d:\d\d:\d\d', bufs[2][1])
+  call assert_match('2019/02/1\d \d\d:\d\d:00', bufs[3][1])
+
+  bwipeout bufa
+  bwipeout bufb
+  bwipeout bufc
+endfunc