vim-patch:8.1.2098: mksession test fails on MS-Windows

Problem:    mksession test fails on MS-Windows.
Solution:   Skip testing with backslashes on MS-Windows.
37ac4a531c

src/nvim/testdir/test_mksession.vim

@@ -336,6 +336,9 @@ endfunc
 
 " Test for changing backslash to forward slash in filenames
 func Test_mksession_slash()
+  if exists('+shellslash')
+    throw 'Skipped: cannot use backslash in file name'
+  endif
   enew
   %bwipe!
   e a\\b\\c