Fix flag tests in src/test/modules/test_oat_hooks
In what must have been a copy'n paste mistake, all the flag tests use
the same flag rather than a different flag each. The bug is not
suprising, considering that it's dead code; add a minimal, testimonial
line to cover it.
This is all pretty inconsequential, because this is just example code,
but it had better be correct.
Discussion: https://postgr.es/m/
20220712152059.fwli2majwgzdmh4r@alvherre.pgsql