Refactor test specific config parsing & generation

The dictionary is generated once during test creation at suite startup.

Change-Id: I4de511d64ccf65d2ec6eec45dcf28529b74c5d26
diff --git a/src/osmo_gsm_tester/testenv.py b/src/osmo_gsm_tester/testenv.py
index d6c18ee..11199c2 100644
--- a/src/osmo_gsm_tester/testenv.py
+++ b/src/osmo_gsm_tester/testenv.py
@@ -146,7 +146,7 @@
         return self.suite_run.config_suite_specific()
 
     def config_test_specific(self):
-        return self.suite_run.config_suite_specific().get(self._test.module_name(), {})
+        return self._test.config_test_specific()
 
     def set_overlay_template_dir(self, template_dir=None):
         '''Overlay a directory on top of default one when looking for