tests: move pwalletMain to wallet test fixture

Scope the variable instead of using an external global; this is how test
fixtures are intended to be used.

Followup to #11713.

src/wallet/test/wallet_test_fixture.cpp

@@ -6,9 +6,6 @@
 
 #include <rpc/server.h>
 #include <wallet/db.h>
-#include <wallet/wallet.h>
-
-std::unique_ptr<CWallet> pwalletMain;
 
 WalletTestingSetup::WalletTestingSetup(const std::string& chainName):
     TestingSetup(chainName)
@@ -27,7 +24,6 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName):
 WalletTestingSetup::~WalletTestingSetup()
 {
     UnregisterValidationInterface(pwalletMain.get());
-    pwalletMain.reset();
 
     bitdb.Flush(true);
     bitdb.Reset();