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.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 168ced4..7af5652 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -565,6 +565,9 @@
 
 " 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