tests: Add incus-user test #395

Job	Run time
Code tests	8m 29s
Client tests (1.20.x, ubuntu-latest)	58s
System tests (1.20.x, cluster, dir)	12m 47s
Client tests (1.20.x, macos-latest)	57s
System tests (1.20.x, cluster, btrfs)	13m 50s
Client tests (1.20.x, windows-latest)	2m 53s
System tests (1.20.x, cluster, lvm)	13m 53s
Documentation tests	9m 2s
System tests (1.20.x, cluster, zfs)	14m 8s
System tests (1.20.x, cluster, ceph)	19m 53s
System tests (1.20.x, cluster, random)	13m 14s
System tests (1.20.x, standalone, dir)	26m 10s
System tests (1.20.x, standalone, btrfs)	22m 43s
System tests (1.20.x, standalone, lvm)	39m 38s
System tests (1.20.x, standalone, zfs)	46m 0s
System tests (1.20.x, standalone, ceph)	1h 16m 12s
System tests (1.20.x, standalone, random)	35m 41s
System tests (stable, cluster, dir)	11m 58s
System tests (stable, standalone, dir)	28m 2s
System tests (tip, cluster, dir)	13m 28s
System tests (tip, standalone, dir)	23m 53s
	7h 13m 49s

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