Prior to this commit, there were several places to potentially add a new
message metadata test:

* `SharedMessageMetadataTests` module
* `MessageEncryptorMetadataTest` class
  (includes `SharedMessageMetadataTests`)
* `MessageEncryptorMetadataMarshalTest` class
  (subclasses `MessageEncryptorMetadataTest`)
* `MessageEncryptorMetadataJSONTest` class
  (subclasses `MessageEncryptorMetadataTest`)
* `MessageEncryptorMetadataJsonWithMarshalFallbackTest` class
  (subclasses `MessageEncryptorMetadataTest`)
* `MessageVerifierMetadataTest` class
  (includes `SharedMessageMetadataTests`)
* `MessageVerifierMetadataMarshalTest` class
  (subclasses `MessageVerifierMetadataTest`)
* `MessageVerifierMetadataJsonWithMarshalFallbackTest` class
  (subclasses `MessageVerifierMetadataTest`)
* `MessageVerifierMetadataJsonTest` class
  (subclasses `MessageVerifierMetadataTest`)
* `MessageVerifierMetadataCustomJSONTest` class
  (subclasses `MessageVerifierMetadataTest`)
* `MessageEncryptorMetadataNullSerializerTest` class
  (subclasses `MessageVerifierMetadataTest`)

This commit refactors the message metadata tests, reducing the list to:

* `MessageMetadataTests` module
* `MessageEncryptorMetadataTest` class (includes `MessageMetadataTests`)
* `MessageVerifierMetadataTest` class (includes `MessageMetadataTests`)

This makes it easier to add new tests, as well as new testing scenarios
(e.g. new encryptor / verifier configurations).

Additionally, this commit fixes two tests that were ineffective:

* The `test_passing_expires_in_less_than_a_second_is_not_expired` test
  (which is now simply a part of the "message expires with :expires_in"
  test) did not use the `with_usec: true` option.  Therefore, the time
  resulting from `travel 0.5.seconds` was truncated, effectively
  traveling 0 seconds.
* The `test_verify_with_use_standard_json_time_format_as_false` test
  (which is now named "expiration works with
  ActiveSupport.use_standard_json_time_format = false") did not specify
  an expiration time.  Therefore, the conditional branch that it was
  supposed to test was never exercised.