test: an organisation delete removes the blobs, Postgres tests skip under -short

TestOrganisationLogo asserted only that the system_files rows go; it now
also waits for the commit and asserts the logo's and avatar's originals and
thumbnails are gone from the bucket. The Postgres tests of the gap closure
call t.Skip under -short instead of returning, so -short runs report them
as skipped, not passed.

access_revocation_test.go

@@ -110,10 +110,10 @@ func wholeSecondsUntruncated(t *testing.T, env *adminEnv, id uint, before time.T
 // revoked subject event per user after the commit; the ban fires only for
 // newly banned users; a failing listener rolls a password reset back; and
 // a banned linked user loses the backend link until unbanned. Postgres
-// only: under -short nothing runs.
+// only: skipped under -short.
 func TestAccessRevocation(t *testing.T) {
 	if testing.Short() {
-		return
+		t.Skip("Postgres")
 	}
 	env := newAdminEnv(t)
 	spy := env.spySubjects()