[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.

test/functional/txn_clone.py

@@ -8,12 +8,6 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import *
 
 class TxnMallTest(BitcoinTestFramework):
-
-    def __init__(self):
-        super().__init__()
-        self.num_nodes = 4
-        self.setup_clean_chain = False
-
     def add_options(self, parser):
         parser.add_option("--mineblock", dest="mine_block", default=False, action="store_true",
                           help="Test double-spend of 1-confirmed transaction")