fuzz: Initialize a full TestingSetup where appropriate

A full TestingSetup is required for both coins_view and
load_external_block_file as they interact with the active chainstate.

src/test/fuzz/coins_view.cpp

@@ -36,9 +36,7 @@ bool operator==(const Coin& a, const Coin& b)
 
 void initialize_coins_view()
 {
-    static const ECCVerifyHandle ecc_verify_handle;
-    ECC_Start();
-    SelectParams(CBaseChainParams::REGTEST);
+    static const auto testing_setup = MakeFuzzingContext<const TestingSetup>();
 }
 
 FUZZ_TARGET_INIT(coins_view, initialize_coins_view)

src/test/fuzz/load_external_block_file.cpp

@@ -15,7 +15,7 @@
 
 void initialize_load_external_block_file()
 {
-    static const auto testing_setup = MakeFuzzingContext<>();
+    static const auto testing_setup = MakeFuzzingContext<const TestingSetup>();
 }
 
 FUZZ_TARGET_INIT(load_external_block_file, initialize_load_external_block_file)