[tests] don't override __init__() in individual tests

Almost all test scripts currently need to override the __init__()
method. When they do that they need to call into super().__init__() as
the base class does some generic initialization.

This commit makes the base class __init__() call into set_test_params()
method. Individual test cases can override set_test_params() to setup
their test parameters.

Github-Pull: #11121
Rebased-From: 5448a1471d

test/functional/rpcnamedargs.py

@@ -10,15 +10,8 @@ from test_framework.util import (
     assert_raises_jsonrpc,
 )
 
-
 class NamedArgumentTest(BitcoinTestFramework):
-    """
-    Test named arguments on RPC calls.
-    """
-
-    def __init__(self):
-        super().__init__()
-        self.setup_clean_chain = False
+    def set_test_params(self):
         self.num_nodes = 1
 
     def run_test(self):