support/tests: use relative test_dir

When we list the available tests, we use test_dir, which is set from
the path of the script. However, when we run the tests, we use the
hard-coded path.

Ditto to find the config file.

For consistency, always use test_dir.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

support/testing/run-tests

@@ -108,8 +108,8 @@ def main():
 
     nose2_args = ["-v",
                   "-N", str(args.testcases),
-                  "-s", "support/testing",
-                  "-c", "support/testing/conf/unittest.cfg"]
+                  "-s", test_dir,
+                  "-c", os.path.join(test_dir, "conf/unittest.cfg")]
 
     if len(args.testname) != 0:
         nose2_args += args.testname