[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/merkle_blocks.py

@@ -8,11 +8,8 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import *
 
 class MerkleBlockTest(BitcoinTestFramework):
-
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.setup_clean_chain = True
-        self.num_nodes = 4
         # Nodes 0/1 are "wallet" nodes, Nodes 2/3 are used for testing
         self.extra_args = [[], [], [], ["-txindex"]]