patch 8.1.0151: mksession test fails on MS-Windows

Problem:    Mksession test fails on MS-Windows.
Solution:   Always use an argument for :lcd.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 49d89ec..d9fe2c2 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -165,9 +165,9 @@
 func Test_mksession_lcd_multiple_tabs()
   tabnew
   tabnew
-  lcd
+  lcd .
   tabfirst
-  lcd
+  lcd .
   mksession! Xtest_mks.out
   tabonly
   source Xtest_mks.out