fix time zone for CustomEmojiService test

What does this MR do?

Set the session timezone at the start of the test.

Otherwise, the timestamps that the test uses for queries are interpreted as UTC (because they have a TZ in the string), but the truncated date is interpreted in the session timezone, which defaults to the server timezone, which can be anything.

Thanks to @fEmber for noticing.

While I was there, I fixed some names that annoyed me.

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