test: Refactor and flatten `BackupFeatureTestCase` #1943

Job	Run time
Build (Unit-tests & Android-Lint)	27m 0s
Android UI-tests on emulator (25, default)	12m 18s
Android UI-tests on emulator (28, default)	8m 45s
Android UI-tests on emulator (27, default)	6h 5m 0s
Android UI-tests on emulator (29, default)	8m 51s
Android UI-tests on emulator (30, default)	10m 28s
Android UI-tests on emulator (31, default)	8m 38s
Android UI-tests on emulator (32, default)	8m 35s
Android UI-tests on emulator (34, default)	8m 49s
Android UI-tests on emulator (26, default)	6h 0m 5s
Android UI-tests on emulator (24, default)	8m 25s
Android UI-tests on emulator (36, default)	9m 38s
Android UI-tests on emulator (33, default)	7m 55s
Android UI-tests on emulator (35, default)	11m 59s
Android UI-tests on emulator (23, default)	9m 35s
	14h 26m 1s

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