Minor code cleanup in dbus-tests.el

Minor code cleanup in dbus-tests.el

* test/lisp/net/dbus-tests.el (dbus--tests-dir): Make it a defconst.
(dbus--test-method-reentry-handler): Mark args as unused.
(dbus-test04-method-reentry): Tag it :expensive-test.  Fix typo.
(dbus-test06-property-types): Remove duplicate test.
(dbus--test-introspect): Use `insert-file-contents-literally'.
(dbus--test-validate-property): Mark expected-annotations as unused.
(dbus--test-validate-m-or-s): Remove superfluous le-clause.