[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/mempool_reorg.py

@@ -13,10 +13,8 @@ from test_framework.util import *
 
 # Create one-input, one-output, no-fee transaction:
 class MempoolCoinbaseTest(BitcoinTestFramework):
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.num_nodes = 2
-        self.setup_clean_chain = False
         self.extra_args = [["-checkmempool"]] * 2
 
     alert_filename = None  # Set by setup_network