test: Create test fixture for wallet

Removes all the `#ifdef ENABLE_WALLET` from `test_bitcoin` by
making the wallet tests use their own fixture.

src/wallet/test/wallet_test_fixture.cpp

@@ -0,0 +1,28 @@
+#include "wallet/test/wallet_test_fixture.h"
+
+#include "rpc/server.h"
+#include "wallet/db.h"
+#include "wallet/wallet.h"
+
+WalletTestingSetup::WalletTestingSetup(const std::string& chainName):
+    TestingSetup(chainName)
+{
+    bitdb.MakeMock();
+
+    bool fFirstRun;
+    pwalletMain = new CWallet("wallet.dat");
+    pwalletMain->LoadWallet(fFirstRun);
+    RegisterValidationInterface(pwalletMain);
+
+    RegisterWalletRPCCommands(tableRPC);
+}
+
+WalletTestingSetup::~WalletTestingSetup()
+{
+    UnregisterValidationInterface(pwalletMain);
+    delete pwalletMain;
+    pwalletMain = NULL;
+
+    bitdb.Flush(true);
+    bitdb.Reset();
+}