CommentTest::testPostIndividualDxWithoutCriticalBaseFields() fails on 8.7 since #2885809

This happens since #2975217: Update the default comment entity owner to the current user. That's commit 0b8908a1d2bdce266620443687ea6e5cfd2058a0. The commit before it passes tests.

I think this difference can be explained by the fact that now the owner is no longer user zero, but the current user.

It'd be really great if this Comment entity weirdness that's documented in both \Drupal\Tests\jsonapi\Functional\CommentTest::testPostIndividualDxWithoutCriticalBaseFields() and \Drupal\Tests\comment\Functional\Rest\CommentResourceTestBase::testPostDxWithoutCriticalBaseFields() would get fixed… but other priorities, I know :(

99.99% certain that #3 fails on 8.6 though. So we'll need to make that change conditionally.

It'd also help if I rolled the patch correctly.

D'oh, #2975217: Update the default comment entity owner to the current user just got reverted. Until that issue lands again, this issue is moot/postponed.

testPostIndividualDxWithoutCriticalBaseFields() is failing again, not because #2975217: Update the default comment entity owner to the current user landed, but because its blocker #2885809: The 'entity_type' and 'field_name' base fields on Comment are required landed!

Re-testing #8 against 8.6 and 8.7.

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