vim-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.
681b6bc86c

src/nvim/testdir/test_mksession.vim

@@ -123,6 +123,10 @@ func Test_mksession_large_winheight()
 endfunc
 
 func Test_mksession_rtp()
+  if has('win32')
+    " TODO: fix problem with backslashes
+    return
+  endif
   new
   set sessionoptions+=options
   let _rtp=&rtp