Disable parallel query in psql error-with-FETCH_COUNT test.

Disable parallel query in psql error-with-FETCH_COUNT test.

The buildfarm members using debug_parallel_query = regress are mostly
unhappy with this test.  I guess what is happening is that rows
generated by a parallel worker are buffered, and might or might not
get to the leader before the expected error occurs.  We did not see
any variability in the old version of this test because each FETCH
would succeed or fail atomically, leading to a predictable number of
rows emitted before failure.  I don't find this to be a bug, just
unspecified behavior, so let's disable parallel query for this one
test case to make the results stable.