Write initial Icedove tests

Since Icedove will soon be in Tails there should be some tests written for it.

Initially, we should probably check that Icedove is configured as per https://tails.boum.org/blueprint/Return_of_Icedove63/#index5h2

Feature Branch: kytv:feature/5663-return-to-icedove

Original created by @kytv on 10332 (Redmine)

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