Stabilize output of new regression test case.

Stabilize output of new regression test case.

The test added by commit 390d58135 turns out to have different output
in CLOBBER_CACHE_ALWAYS builds: there's an extra CONTEXT line in the
error message as a result of detecting the error at a different place.
Possibly we should do something to make that more consistent.  But as
a stopgap measure to make the buildfarm green again, adjust the test
to suppress CONTEXT entirely.  We can revert this if we do something
in the backend to eliminate the inconsistency.

Discussion: https://postgr.es/m/31545.1512924904@sss.pgh.pa.us