De-duplicate InheritanceManager3Test tests.

The duplication that's being eliminated was between InheritanceManager3_2Test and InheritanceManager3Test; these classes were split apart as part of implementing null safety. Now that null safety is always enabled, the split is no longer needed.

Change-Id: I1efa98ce49b0845808d3d2898120a960eff45c6e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420844
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: scheglov