[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/keypool.py

@@ -8,6 +8,8 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import *
 
 class KeyPoolTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
 
     def run_test(self):
         nodes = self.nodes
@@ -78,10 +80,5 @@ class KeyPoolTest(BitcoinTestFramework):
         assert_equal(wi['keypoolsize_hd_internal'], 100)
         assert_equal(wi['keypoolsize'], 100)
 
-    def __init__(self):
-        super().__init__()
-        self.setup_clean_chain = False
-        self.num_nodes = 1
-
 if __name__ == '__main__':
     KeyPoolTest().main()