ctrl test: fix: adjust expectations after stricter ctrl parsing

After libosmocore I96a9b6b6a3a5e0b80513aa9eaa727ae8c9c7d7a1 the CTRL interface
returns stricter errors. Adjust the expectations of
test_subscriber_errors.ctrl to fix the external tests on master.

Change-Id: I9337b6b4f3fa8822c91760deb01f18a77a073d19
1 file changed