Improve single comment endpoint read permissions checks

[adamsilverstein]

Trac ticket: https://core.trac.wordpress.org/ticket/44157

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[github-actions]

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Props adamsilverstein, peterwilsoncc, westonruter, jeffpaul.

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[github-actions]

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[peterwilsoncc]

The various assertions could do with assertSame and for the tests with multiple assertions a different message for each assertion.

[adamsilverstein]

@peterwilsoncc I went ahead and committed what we have. I would be in favor of backporting this to 6.9, what do you think?

[jeffpaul]

FWIW I support backporting to 6.9, but defer to a proper committer review/approval there.

[westonruter]

Committed in r61276.

Backported to 6.9 branch in r61297.