patch 8.1.0816: test for 'runtimepath' in session fails on MS-Windows

Problem:    Test for 'runtimepath' in session fails on MS-Windows.
Solution:   Skip the test for now.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 6797ad9..3698976 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -127,6 +127,10 @@
 endfunc
 
 func Test_mksession_rtp()
+  if has('win32')
+    " TODO: fix problem with backslashes
+    return
+  endif
   new
   let _rtp=&rtp
   " Make a real long (invalid) runtimepath value,