[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/getchaintips.py

@@ -14,13 +14,7 @@ from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import assert_equal
 
 class GetChainTipsTest (BitcoinTestFramework):
-    def __init__(self):
-        super().__init__()
-        self.num_nodes = 4
-        self.setup_clean_chain = False
-
     def run_test (self):
-
         tips = self.nodes[0].getchaintips ()
         assert_equal (len (tips), 1)
         assert_equal (tips[0]['branchlen'], 0)