Use dict.items and remove unused variables in tests

[Flamefire]

This resolves a few warnings generated by PyLint which make the tests a bit simpler/clearer.

E.g. stdout, stderr = self.run_main(args, raise_error=True) is used but then the variables are never used.
When reading this code again it is unclear if this is intentional

Using dict.items is shorter and makes it easier to see what the dict is made of, e.g.:

-                for key in tcs:
-                    tcname = key.split('@')[0]
-                    tcversion, generic_flags, custom_tcopts = tcs[key]  # Could be way below
+                for key, (tcversion, generic_flags, custom_tcopts) in tcs.items():
+                    tcname = key.split('@')[0]