patch 9.0.1654: MS-Windows: test for default 'viewdir' fails

Problem:    MS-Windows: test for default 'viewdir' fails.
Solution:   Escape the pattern.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 0c4838e..aa29182 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -1265,7 +1265,8 @@
 " Test default 'viewdir' value
 func Test_mkview_default_home()
   if has('win32')
-    call assert_match('^' .. $ORIGHOME .. '/vimfiles', &viewdir)
+    " use escape() to handle backslash path separators
+    call assert_match('^' .. escape($ORIGHOME, '\') .. '/vimfiles', &viewdir)
   elseif has('unix')
     call assert_match('^' .. $ORIGHOME .. '/.vim', &viewdir)
   elseif has('amiga')