fuzz: Consolidate fuzzing TestingSetup initialization

Previously, the {Basic,}TestingSetup for fuzzers were set up in many ways:

1. Calling InitializeFuzzingContext, which implicitly constructs a static
   const BasicTestingSetup
2. Directly constructing a static const BasicTestingSetup in the initialize_*
   function
3. Directly constructing a static TestingSetup and reproducing the
   initialization arguments (I'm assuming because
   InitializeFuzzingContext only initializes a BasicTestingSetup)

The new, relatively-simple MakeFuzzingContext function allows us to
consolidate these methods of initialization by being flexible enough to
be used in all situations. It:

1. Is templated so that we can choose to initialize any of
   the *TestingSetup classes
2. Has sane defaults which are often used in fuzzers but are also
   easily overridable
3. Returns a unique_ptr, explicitly transferring ownership to the caller
   to deal with according to its situation

src/test/fuzz/policy_estimator_io.cpp

@@ -12,7 +12,7 @@
 
 void initialize_policy_estimator_io()
 {
-    InitializeFuzzingContext();
+    static const auto testing_setup = MakeFuzzingContext<>();
 }
 
 FUZZ_TARGET_INIT(policy_estimator_io, initialize_policy_estimator_io)