test: Rename wallet.dat to wallet_test.dat

Indicate that the file name is not hardcoded, and a little bit of safety
so that it never nukes the main wallet.

Suggestion by Marco Falke.

src/wallet/test/wallet_test_fixture.cpp

@@ -10,7 +10,7 @@ WalletTestingSetup::WalletTestingSetup(const std::string& chainName):
     bitdb.MakeMock();
 
     bool fFirstRun;
-    pwalletMain = new CWallet("wallet.dat");
+    pwalletMain = new CWallet("wallet_test.dat");
     pwalletMain->LoadWallet(fFirstRun);
     RegisterValidationInterface(pwalletMain);