Write initial Icedove tests

Originally created by @kytv on #10332 (Redmine)

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_Icedove__63__/#index5h2

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

Edited May 15, 2020 by kytv

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