patch 8.1.0807: session test fails on MS-Windows

Problem:    Session test fails on MS-Windows.
Solution:   Don't try creating file with illegal name.
diff --git a/src/testdir/test_mksession.vim b/src/testdir/test_mksession.vim
index 145c91c..93a68ad 100644
--- a/src/testdir/test_mksession.vim
+++ b/src/testdir/test_mksession.vim
@@ -427,6 +427,10 @@
 endfunc
 
 func Test_mksession_quote_in_filename()
+  if !has('unix')
+    " only Unix can handle this weird filename
+    return
+  endif
   let v:errmsg = ''
   %bwipe!
   split another