patch 9.1.0721: tests: test_mksession does not consider XDG_CONFIG_HOME

Problem:  tests: test_mksession does not consider XDG_CONFIG_HOME
Solution: allow to match $HOME/.vim/ and $HOME/.config/vim for &viewdir
          (John M Devin)

closes: #15639

Signed-off-by: John M Devin <john.m.devin@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 914d1e3..d0ee6e8 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -1270,7 +1270,10 @@
     " use escape() to handle backslash path separators
     call assert_match('^' .. escape($ORIGHOME, '\') .. '/vimfiles', &viewdir)
   elseif has('unix')
-    call assert_match('^' .. $ORIGHOME .. '/.vim', &viewdir)
+    call assert_match(
+          \ '^' .. $ORIGHOME .. '/.vim\|' ..
+          \ '^' .. $XDG_CONFIG_HOME .. '/vim'
+          \ , &viewdir)
   elseif has('amiga')
     call assert_match('^home:vimfiles', &viewdir)
   elseif has('mac')