tests: add qdb_watch test
tests: add qdb_watch test

Use dom0 instance of QubesDB, to be independent of working (or not) VMs
at this stage.

QubesOS/qubes-issues#2940

QubesOS/qubes-core-admin,woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin