[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/mempool_packages.py

@@ -12,10 +12,8 @@ MAX_ANCESTORS = 25
 MAX_DESCENDANTS = 25
 
 class MempoolPackagesTest(BitcoinTestFramework):
-    def __init__(self):
-        super().__init__()
+    def set_test_params(self):
         self.num_nodes = 2
-        self.setup_clean_chain = False
         self.extra_args = [["-maxorphantx=1000"], ["-maxorphantx=1000", "-limitancestorcount=5"]]
 
     # Build a transaction that spends parent_txid:vout