analyzer: Tidy the invalidUseOfVisibleForTestingMember tests

These test cases were quite "non-modern." I just:

* removed use of `main`
* removed unused fields
* put the main library file, with the visibleForTesting member, in
  the package's `lib` directory
* used `assert(No)ErrorsInCode` when the test file is in `lib` (the
  main test case).
* removed non-idiomatic `new`
* do not assert on diagnostics in uninteresting files.

Change-Id: I39281b327022708cefb67cff7dd172de5a084bd3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/451664
Reviewed-by: Konstantin Shcheglov <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>

Author: srawlins