Add Tramp tests

* lisp/net/tramp.el (tramp-syntax): Adapt docstring.

* test/lisp/net/tramp-tests.el
(tramp-test01-file-name-syntax-simplified)
(tramp-test01-file-name-syntax-separate)
(tramp-test02-file-name-dissect-simplified)
(tramp-test02-file-name-dissect-separate): New tests.

lisp/net/tramp.el

@@ -669,7 +669,10 @@ It can have the following values:
 
   `default'    -- Default syntax
   `simplified' -- Ange-FTP like syntax
-  `separate'   -- Syntax as defined for XEmacs originally."
+  `separate'   -- Syntax as defined for XEmacs originally
+
+Do not change the value by `setq', it must be changed only by
+`custom-set-variables'.  See also `tramp-change-syntax'."
   :group 'tramp
   :version "26.1"
   :package-version '(Tramp . "2.3.2")