Resolve Venus config files relative to test files, instead of CWD

Merging Adrian's fix for issue #164.

This change allows us to correctly resolve config files relative to the test file, rather than the current working directory where Venus is run from.

For example, say you have the following directory structure:

```bash
.
└── project
    └── test
        ├── specs
        │   └── control.spec.js
        └── .venus
            ├── adaptors
            │   └── .gitignore
            ├── config
            ├── libraries
            │   └── .gitignore
            └── templates
                └── .gitignore
```

Before this fix, if you ran `venus run -t project/test/specs/control.spec.js`, the config file in `project/test/.venus/config` would be ignored.

With this fix, the config file is correctly found since it is contained in a `.venus` folder above the location of the test file.