tests dir has been moved to the testsuite submodule:

* the tests dir has been removed from the main gpac repo
* a testsuite repo has been added as a submodule
* the testsuite repo contains the test scripts, media and reference
hashes
* additional media are being downloaded by the make_tests script but
should be moved to git lfs in the future - make sure you have git-lfs
installed (https://help.github.com/en/articles/installing-git-large-file-storage)
* since the gpac repo now contains a submodule, remember to run git
submodule update (--init) when pulling if you want to use the test suite
* conversly, changes to the test suite now require a commit/push to the
testsuite repo **and** commit/push of the main repo to take the changes
in submodule into account

.gitmodules

@@ -0,0 +1,5 @@
+[submodule "testsuite"]
+	path = testsuite
+	url = https://github.com/gpac/testsuite.git
+	branch = filters
+	ignore = dirty