[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/p2p-feefilter.py

@@ -37,11 +37,8 @@ class TestNode(NodeConnCB):
             self.txinvs = []
 
 class FeeFilterTest(BitcoinTestFramework):
-
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.num_nodes = 2
-        self.setup_clean_chain = False
 
     def run_test(self):
         node1 = self.nodes[1]