Merge #20946: fuzz: Consolidate fuzzing TestingSetup initialization

abb6fa7285 fuzz: Initialize a full TestingSetup where appropriate (Carl Dong)
713314abfa fuzz: Consolidate fuzzing TestingSetup initialization (Carl Dong)

Pull request description:

  ```
  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
  ```

  ~~Question for fuzzing people: was it intentional that `src/test/fuzz/net.cpp` would directly instantiate the `BasicTestingSetup` and thus omit the `"-nodebuglogfile"` flag?~~ [Answered](https://github.com/bitcoin/bitcoin/pull/20946#issuecomment-761537108)

ACKs for top commit:
  MarcoFalke:
    ACK abb6fa7285

Tree-SHA512: 96a5ca6f4cd5ea0e9483b60165b31ae3e9003918c700a7f6ade48010f419f2a6312e10b816b3187f1d263798827571866e4c4ac0bbfb2e0c79dfad254cda68e7

src/test/fuzz/process_messages.cpp

@@ -17,18 +17,14 @@
 #include <validation.h>
 #include <validationinterface.h>
 
+namespace {
 const TestingSetup* g_setup;
+} // namespace
 
 void initialize_process_messages()
 {
-    static TestingSetup setup{
-        CBaseChainParams::REGTEST,
-        {
-            "-nodebuglogfile",
-        },
-    };
-    g_setup = &setup;
-
+    static const auto testing_setup = MakeFuzzingContext<const TestingSetup>();
+    g_setup = testing_setup.get();
     for (int i = 0; i < 2 * COINBASE_MATURITY; i++) {
         MineBlock(g_setup->m_node, CScript() << OP_TRUE);
     }