Fix test case from
b0c5b215d.
I'd not checked that this iteration of the test actually worked
with a bootstrap superuser not named 'postgres'. It didn't,
because the coercion rules for CASE caused us to try to cast
the 'postgres' literal to regrole. Mea culpa.
Per buildfarm (via Alexander Korotkov)
Discussion: https://postgr.es/m/CAPpHfdsV=iTvH6B858hnH1bLgewYH6cdTnO_eOOw9EOa8kehkA@mail.gmail.com