This trait is being used already to test MessageValue serialization, but adhoc tests in MessageTest are used for Message serialization.
Using MessageSerializatioTestTrait requires some changes to the Message class to make it testable that way, which might be a bit risky. (It's also possible that switching to json-codec might make it easier to deal with (eg) Title objects embedded in the Message.)
Change #1068290 had a related patch set uploaded (by C. Scott Ananian; author: Bartosz Dziewoński):
[mediawiki/core@master] [WIP] Message: Add more tests using SerializationTestTrait
Change #1064425 had a related patch set uploaded (by C. Scott Ananian; author: Bartosz Dziewoński):
[mediawiki/core@master] Message: Add more tests for serialization and unserialization
Change #1064425 merged by jenkins-bot:
[mediawiki/core@master] Message: Add more tests for serialization and unserialization
Change #1102760 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):
[mediawiki/core@master] Message: Delay constructing Language objects
Change #1102761 had a related patch set uploaded (by Bartosz Dziewoński; author: Bartosz Dziewoński):
[mediawiki/core@master] Message: Remove distinction between 'titlestr' and 'titlevalue'
Change #1102760 merged by jenkins-bot:
[mediawiki/core@master] Message: Delay constructing Language objects
Change #1102761 merged by jenkins-bot:
[mediawiki/core@master] Message: Remove distinction between 'titlestr' and 'titlevalue'
Change #1068290 merged by jenkins-bot:
[mediawiki/core@master] Message: Add more tests using SerializationTestTrait