Primarily addresses https://github.com/dotnet/roslyn/issues/29519#issuecomment-420067012

```
1. Enable the rules be default
2. Remove all the workaround/hacks for disabling the rules for product and enabling for tests.
3. Address offline feedback from @sharwell about avoiding deriving the tests from `TestBase`.
4. Separate out the pending non-design related work items from first comment into separate issues.
```

For 4., I have repurposed https://github.com/dotnet/roslyn/issues/29519 to track the pending issues with checkboxes in the first issue comment.