more clarity for example test setup

In case the changes I made are not obvious:

*   **I reset the `path.base` config variable in `getEnvironmentSetUp()`**

    The way it was, the base path was `/path/to/project/workbench/myproject/mypackage/vendor/orchestra/testbench/src/fixture` which meant that in order for a package developer to set the path to their own migrations, the `--path` setting for the migration had to be something like `../../../../../tests/migrations` which I found non-obvious and a bit cumbersome.

*   **I added some commented out suggestions for potentially common testing scenarios**

    I think the `DatabaseFixtureTest` will still run successfully as-is, but I added some commented out sections that mirror what I'm doing in the test for the package I'm developing right now.  I think others might find them useful as well.  I'm relatively new to Laravel.  Just started my first project about two weeks ago, so I'm still figuring out how the whole thing fits together.

That's it.  Hope this helps!