[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/invalidtxrequest.py

@@ -19,9 +19,9 @@ class InvalidTxRequestTest(ComparisonTestFramework):
 
     ''' Can either run this test as 1 node with expected answers, or two and compare them. 
         Change the "outcome" variable from each TestInstance object to only do the comparison. '''
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.num_nodes = 1
+        self.setup_clean_chain = True
 
     def run_test(self):
         test = TestManager(self, self.options.tmpdir)