tests: Reduce calls to InitS*Cache()

In src/test/fuzz/script_sigcache.cpp, we should really be setting up a
full working BasicTestingSetup. The initialize_ function is only run
once anyway.

In src/test/txvalidationcache_tests.cpp, the Dersig100Setup inherits
from BasicTestingSetup, which should have already set up a global script
execution cache without the need to explicitly call
InitScriptExecutionCache.

src/test/txvalidationcache_tests.cpp

@@ -161,11 +161,6 @@ BOOST_FIXTURE_TEST_CASE(checkinputs_test, Dersig100Setup)
 {
     // Test that passing CheckInputScripts with one set of script flags doesn't imply
     // that we would pass again with a different set of flags.
-    {
-        LOCK(cs_main);
-        InitScriptExecutionCache();
-    }
-
     CScript p2pk_scriptPubKey = CScript() << ToByteVector(coinbaseKey.GetPubKey()) << OP_CHECKSIG;
     CScript p2sh_scriptPubKey = GetScriptForDestination(ScriptHash(p2pk_scriptPubKey));
     CScript p2pkh_scriptPubKey = GetScriptForDestination(PKHash(coinbaseKey.GetPubKey()));