Fix 2 tests that fail in MS-Windows

Fix 2 tests that fail in MS-Windows

https://lists.gnu.org/archive/html/emacs-devel/2017-08/msg00018.html
* test/lisp/vc/ediff-ptch-tests.el (ediff-ptch-test-bug26084):
Add comments to explain the test logic.
Pass '--binary' option to 'patch' program in windows environments.
Check explicitely that a backup is created before compare file contents.

* test/lisp/dired-tests.el (dired-test-bug25609):
Declare variable 'dired-dwim-target' right before the test.
Add comments to explain the test logic.
Ensure, before test the bug condition, that we are displaying the
2 dired buffers created in this test, and no other dired buffer
is shown.