further refactoring for CNAME tests and fixed an error of an existing test.
The fix is for MXAlias: it didn't test what was to be tested: the intended case is where MX exchange name is an alias: cnamemx.example.com. MX 10 cnamemailer.example.com. cnamemailer.example.com. CNAME www.example.com. (and the test asks for cnamemx.example.com/MX) but it tested the case where the owner name of MX is an alias (ask cnamemailer.example.com/MX in the above example).
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