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.h

@@ -0,0 +1,18 @@
+// Copyright (c) 2016 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_WALLET_TEST_FIXTURE_H
+#define BITCOIN_WALLET_TEST_FIXTURE_H
+
+#include "test/test_bitcoin.h"
+
+/** Testing setup and teardown for wallet.
+ */
+struct WalletTestingSetup: public TestingSetup {
+    WalletTestingSetup(const std::string& chainName = CBaseChainParams::MAIN);
+    ~WalletTestingSetup();
+};
+
+#endif
+