[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/proxy_test.py

@@ -41,13 +41,7 @@ from test_framework.netutil import test_ipv6_local
 
 RANGE_BEGIN = PORT_MIN + 2 * PORT_RANGE  # Start after p2p and rpc ports
 
-
 class ProxyTest(BitcoinTestFramework):
-    def __init__(self):
-        super().__init__()
-        self.num_nodes = 4
-        self.setup_clean_chain = False
-
     def setup_nodes(self):
         self.have_ipv6 = test_ipv6_local()
         # Create two proxies on different ports