Refactor use_of_void tests

* Merge pre-NNBD tests with other tests; remove duplicates.
* Avoid using `main` in tests. Avoid declaring an unused variable.
* Change expectations of using void the left or right side of a
  conditional expression. These are explicitly allowed by the spec.
* Remove some comments about strong mode.

Change-Id: Iad9f3c37bde83b29ba82b0d671659a3330ca151e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/337843
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>