[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/create_cache.py

@@ -12,11 +12,9 @@ tests are being run in parallel.
 from test_framework.test_framework import BitcoinTestFramework
 
 class CreateCache(BitcoinTestFramework):
+    # Test network and test nodes are not required:
 
-    def __init__(self):
-        super().__init__()
-
-        # Test network and test nodes are not required:
+    def set_test_params(self):
         self.num_nodes = 0
 
     def setup_network(self):