[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/example_test.py

@@ -73,15 +73,11 @@ def custom_function():
 class ExampleTest(BitcoinTestFramework):
     # Each functional test is a subclass of the BitcoinTestFramework class.
 
-    # Override the __init__(), add_options(), setup_chain(), setup_network()
+    # Override the set_test_params(), add_options(), setup_chain(), setup_network()
     # and setup_nodes() methods to customize the test setup as required.
 
-    def __init__(self):
-        """Initialize the test
-
-        Call super().__init__() first, and then override any test parameters
-        for your individual test."""
-        super().__init__()
+    def set_test_params(self):
+        """Override any test parameters for your individual test."""
         self.setup_clean_chain = True
         self.num_nodes = 3
         # Use self.extra_args to change command-line arguments for the nodes