test: accept GC'ed units in newer LVM
Since lvm 2.03.15 the transient units are started without `-r`, thus
disappearing once they finish and breaking the test (which expects them
to remain loaded after finishing). Let's accept `LoadState=not-found` as
a valid result as well to fix this.
Follow-up to:
d10d562bd4b9f93130fb2b23f2b0d0d4126ea7d4
See: https://sourceware.org/git/?p=lvm2.git;a=commit;h=
fbd8b0cf43dc67f51f86f060dce748f446985855
(cherry picked from commit
b034f02c628057c30a2136289a1b388a6fb9a737)