dired-tests: Unload tested features after test them

dired-tests: Unload tested features after test them

Some tests are for Dired with ls-lisp or eshell-ls.
Requiring these features add an advice on `dired' and
might affect other tests.
Do not require these features at the top of the file; require
then inside the tests and unload then at the end.
* test/lisp/dired-tests.el (dired-test-bug27693)
(dired-test-bug7131, dired-test-bug27817, dired-test-bug27631):
require ls-lisp and/or eshell-ls inside the test; unload the
features at the end.