[NFC] Bring back deleted activity email tests

[demeritcowboy]

Overview

These tests were deleted in #23571 but they should have been migrated back when the function was deprecated.

The testSendEmailWithMultipleToRecipients may or may not still apply so have left out for the moment. Would need to do more research.

[civibot]

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[eileenmcnaughton]

@demeritcowboy - so this makes the tests test the code we actually use instead of the deprecated code we no longer use? Sounds like a win

[demeritcowboy]

I get what you're saying - there's a longstanding debate in the computer science world about whether to test internal code. I myself have mixed feelings about it. I'll note \Civi\Test\Invasive exists for this reason. Of course the answer is: "sometimes".

But I think it's not crazy to have a rule that when coverage already exists, then if the covered code blocks are moved then whatever it is those tests are covering should stay covered. I admit I don't know of a tool to help with that, so I don't know how to automate enforcing such a rule. It seems like a loophole in unit-testing.

[eileenmcnaughton]

@demeritcowboy this PR creates more testing - that's a good thing!