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.