sanitize tests/run-tests.el

* don't use defvar when let is enough
* use built-in functions to create paths
* test whether dirs exist before adding to load-path
tests/run-tests.el: add /path/to/magit/../{mocker,cl-lib} to load-path

It is quite reasonable to expect the directories containing the
dependencies to be located in the same directory as the magit
directory is located in.  At least this is the case when manually
installing each package as a subdirectory of some "site-lisp"
directory.  Or when using submodules as I do.
Merge pull request #642 from tarsius/tests

tests/run-tests.el: add /path/to/magit/../{mocker,cl-lib} to load-path