Use fake mounter in tests #3121

Job	Run time
detect	1s
build-toolkit	2m 34s
build-matrix (blue) / build-os	2m 51s
build-matrix (green) / build-os	3m 27s
build-matrix (blue) / detect	9s
build-matrix (green) / detect	11s
build-matrix (orange) / build-os	3m 57s
build-matrix (orange) / detect	9s
build-matrix (blue) / build-iso	1m 41s
build-matrix (blue) / build-disk	2m 9s
build-matrix (green) / build-iso	1m 44s
build-matrix (green) / build-disk	2m 18s
build-matrix (orange) / build-iso	2m 36s
build-matrix (orange) / build-disk	3m 4s
build-matrix (blue) / test-installer	3m 49s
build-matrix (green) / test-installer	3m 45s
build-matrix (blue) / tests-matrix (test-active)	2m 37s
build-matrix (green) / tests-matrix (test-upgrade)	3m 52s
build-matrix (green) / tests-matrix (test-downgrade)	5m 44s
build-matrix (green) / tests-matrix (test-recovery)	9m 56s
build-matrix (green) / tests-matrix (test-fallback)	9m 47s
build-matrix (green) / tests-matrix (test-fsck)	4m 50s
build-matrix (green) / tests-matrix (test-grubfallback)	4m 15s
build-matrix (orange) / test-installer	4m 31s
build-matrix (orange) / tests-matrix (test-active)	3m 22s
	1h 23m 19s

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