Replace usage of assertAttributeInstanceOf() that is deprecated

Agree, this is not testing anything useful.

Well this is testing the setter injection in \Drupal\Core\Entity\EntityTypeManager::createHandlerInstance().

I think we could make the $moduleHandler and$stringTranslation properties public in the test classes and test them.

All instances of assertAttributeInstanceOf() have been replaced.
The creation of the class TestEntityHandlerBase has been done to test testGetHandler().
The classes TestEntityForm and TestRouteProvider have 2 extra method for testing.
The removal of suppressing of the warning for assertAttributeInstanceOf() makes sure that all instances have been removed.
All code changes look good to me.
For me it is RTBC.

So the reason I suggested making the params public rather than adding methods is for the following reasons
1. We closer to the original tests without adding functionality.
2. The documentation can be

/**
 * {@inheritdoc}
 */

3. We don't have to ponder why you've overridden \Drupal\Core\StringTranslation\StringTranslationTrait::getStringTranslation() and not \Drupal\Core\Entity\EntityHandlerBase::moduleHandler()
4. And if the upstream signatures change we do't have to worry about anything.

I should have documented this before.

Looks good.

Committed and pushed de981ef81e to 9.1.x and de686a9327 to 9.0.x. Thanks!