[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.

Github-Pull: #11121
Rebased-From: 5448a1471d

test/functional/pruning.py

@@ -26,9 +26,7 @@ def calc_usage(blockdir):
     return sum(os.path.getsize(blockdir+f) for f in os.listdir(blockdir) if os.path.isfile(blockdir+f)) / (1024. * 1024.)
 
 class PruneTest(BitcoinTestFramework):
-
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 6