Refactoring database_fixture.cpp to remove test case names.

**User Story**
Create a new fixture class that is derived from database_fixture, add a hook function there, every individual test case that uses that fixture can define their own parameters for initializing the app instance.
Reference:
https://github.com/bitshares/bitshares-core/pull/1903#discussion_r313486460
**Impacts**
Describe which portion(s) of BitShares Core may be impacted by your request. Please tick at least one box.
- [ ] API (the application programming interface)
- [X] Build (the build process or something prior to compiled code)
- [ ] CLI (the command line wallet)
- [ ] Deployment (the deployment process after building such as Docker, Travis, etc.)
- [ ] DEX (the Decentralized EXchange, market engine, etc.)
- [ ] P2P (the peer-to-peer network for transaction/block propagation)
- [ ] Performance (system or user efficiency, etc.)
- [ ] Protocol (the blockchain logic, consensus, validation, etc.)
- [ ] Security (the security of system or user data, etc.)
- [ ] UX (the User Experience)
- [ ] Other (please add below)

**Additional Context (optional)**
Add any other context about your request here.

## CORE TEAM TASK LIST
- [ ] Evaluate / Prioritize Feature Request
- [ ] Refine User Stories / Requirements
- [ ] Define Test Cases
- [ ] Design / Develop Solution
- [ ] Perform QA/Testing
- [ ] Update Documentation

Start by reading database_fixture.cpp and the discussion in pull request 1903, then locate the test cases that use database_fixture. Determine the fixture initialization behavior and the required hook parameters from that discussion. Done means the fixture no longer depends on test case names and affected tests still build and pass.

Written by the indexing model from the issue text.