[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/getblocktemplate_longpoll.py

@@ -23,11 +23,6 @@ class LongpollThread(threading.Thread):
         self.node.getblocktemplate({'longpollid':self.longpollid})
 
 class GetBlockTemplateLPTest(BitcoinTestFramework):
-    def __init__(self):
-        super().__init__()
-        self.num_nodes = 4
-        self.setup_clean_chain = False
-
     def run_test(self):
         self.log.info("Warning: this test will take about 70 seconds in the best case. Be patient.")
         self.nodes[0].generate(10)