Fix plpgsql tests for debug_invalidate_system_caches_always.

Commit c9d529848 resulted in having a couple more places where
the error context stack for a failure varies depending on
debug_invalidate_system_caches_always (nee CLOBBER_CACHE_ALWAYS).
This is not very surprising, since we have to re-parse cached
plans if the plan cache is clobbered.  Stabilize the expected
test output by hiding the context stack in these places,
as we've done elsewhere in this test script.

(Another idea worth considering, now that we have
debug_invalidate_system_caches_always, is to force it to zero for
these test cases.  That seems like it'd risk reducing the coverage
of cache-clobber testing, which might or might not be worth being
able to verify that we get the expected error output in normal
cases.  For the moment I just stuck with the existing technique.)

In passing, update comments that referred to CLOBBER_CACHE_ALWAYS.

Per buildfarm member hyrax.

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