This commit factors tests related to `ActiveSupport::Messages::Rotator`
into `MessageEncryptorRotatorTest` and `MessageVerifierRotatorTest`
classes, with a shared `MessageRotatorTests` module.  In particular,
this:

* Replaces the `test_rotating_serializer` test in the
  `MessageEncryptorTest` class because it did not actually rotate the
  serializer.

* Removes the `test_on_rotation_is_called_and_returns_modified_messages`
  test from the `MessageEncryptorWithJsonSerializerTest` class because
  its only purpose is to override a test of the same name in the
  `MessageEncryptorTest` parent class, because the expected value must
  use string keys instead of symbol keys due to the serializer.

* Removes the `test_on_rotation_can_be_passed_at_the_constructor_level`
  test from the `MessageEncryptorWithJsonSerializerTest` class for the
  same reason as above.

* Removes the `test_on_rotation_option_takes_precedence_over_the_one_given_in_constructor`
  test from the `MessageEncryptorWithJsonSerializerTest` class for the
  same reason as above.

* Removes the `test_on_rotation_is_called_and_verified_returns_message`
  test from the `JsonSerializeMarshalFallbackMessageVerifierTest` class
  because it is rotating the secret and digest, and is not specific to
  the serializer.

* Removes the `test_on_rotation_is_called_and_verified_returns_message`
  test from the `DefaultMarshalSerializerMessageVerifierTest` class for
  the same reason as above.

* Adds thorough test coverage for the `on_rotation` option for both
  `MessageVerifier` and `MessageEncryptor` (previous test coverage was
  mostly just for `MessageEncryptor`), including coverage for when the
  `on_rotation` proc should _not_ be called.

* Adds test coverage for rotating the `url_safe` option, for both
  `MessageVerifier` and `MessageEncryptor`.