test: cover the referral signal and the configuration libnvme refuses

[martin-belanger]

Two paths in ctrl.py that the nvmet-based coverage run cannot reach.

A discovery controller tells the service when the set of referrals in its log pages changes, and stafd reconfigures its controllers on that signal, so it matters that it fires when the list changes and stays quiet when it does not. nvmet serves no referrals at all, so nothing exercised either half.

The other is the configuration libnvme will not build a controller from

• an incomplete host identity, most likely. There is nothing to retry until the configuration changes, and the retry timer is how we find out that it has; the tests pin that the exception is caught rather than escaping, and that the timer is armed at the slow period.

No behaviour changes. ctrl.py goes from 96% to 97%, and the whole from 94% to 95%. What is left in ctrl.py is mostly the DIM registration exchange, which needs a Centralized Discovery controller to talk to.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.86%. Comparing base (d901098) to head (a3e2152).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #516      +/-   ##
==========================================
+ Coverage   70.56%   70.86%   +0.29%     
==========================================
  Files          16       16              
  Lines        2742     2742              
==========================================
+ Hits         1935     1943       +8     
+ Misses        807      799       -8     

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