[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/resendwallettransactions.py

@@ -8,11 +8,9 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal, assert_raises_jsonrpc
 
 class ResendWalletTransactionsTest(BitcoinTestFramework):
-
-    def __init__(self):
-        super().__init__()
-        self.extra_args = [['--walletbroadcast=false']]
+    def set_test_params(self):
         self.num_nodes = 1
+        self.extra_args = [['--walletbroadcast=false']]
 
     def run_test(self):
         # Should raise RPC_WALLET_ERROR (-4) if walletbroadcast is disabled.