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_bufwintabinfo.vim b/src/testdir/test_bufwintabinfo.vim
index ee22ebd..b4b8a10 100644
--- a/src/testdir/test_bufwintabinfo.vim
+++ b/src/testdir/test_bufwintabinfo.vim
@@ -139,3 +139,15 @@
     set foldlevel=0
   endif
 endfunc
+
+function Test_getbufinfo_lastused()
+  call test_settime(1234567)
+  edit Xtestfile1
+  enew
+  call test_settime(7654321)
+  edit Xtestfile2
+  enew
+  call assert_equal(getbufinfo('Xtestfile1')[0].lastused, 1234567)
+  call assert_equal(getbufinfo('Xtestfile2')[0].lastused, 7654321)
+  call test_settime(0)
+endfunc