[test_framework] Set PortSeed.n directly after initialising params

This allows us to use `p2p_port()` with `set_test_params()`.

test/functional/test_framework/test_framework.py

@@ -223,11 +223,11 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
                 # It still needs to exist and be None in order for tests to work however.
                 self.options.descriptors = None
 
+        PortSeed.n = self.options.port_seed
+
     def setup(self):
         """Call this method to start up the test framework object with options set."""
 
-        PortSeed.n = self.options.port_seed
-
         check_json_precision()
 
         self.options.cachedir = os.path.abspath(self.options.cachedir)