Fix new Object Access hooks test

Fix new Object Access hooks test

Commit 90efa2f556 caused some issues with EXEC_BACKEND builds and with
force_parallel_mode = regress setups. For the first issue we no longer
test if the module has been preloaded, and in fact we don't preload it,
but simply LOAD it in the test script. For the second issue we suppress
error messages emanating from parallel workers.

Mark Dilger

Discussion: https://postgr.es/m/7f6d54a1-4024-3b6e-e3ec-26cd394aac9e@dunslane.net