Run BDB disk-less for test_bitcoin

src/db.h

@@ -35,6 +35,7 @@ class CDBEnv
 private:
     bool fDetachDB;
     bool fDbEnvInit;
+    bool fMockDb;
     boost::filesystem::path pathEnv;
 
     void EnvShutdown();
@@ -47,6 +48,8 @@ public:
 
     CDBEnv();
     ~CDBEnv();
+    void MakeMock();
+    bool IsMock() { return fMockDb; };
     bool Open(boost::filesystem::path pathEnv_);
     void Close();
     void Flush(bool fShutdown);