Tighten test_predtest's input checks, and improve error messages.

Tighten test_predtest's input checks, and improve error messages.

test_predtest() neglected to consider the possibility that
SPI_plan_get_cached_plan would return NULL.  This led to a core
dump if the input (incorrectly) contains more than one SQL
command.

While here, let's expend more than zero effort on the error
message for this case and nearby ones.

Per (half of) bug #18483 from Alexander Kozhemyakin.
Back-patch to all supported branches, not because this is
very significant (it's merely test scaffolding) but to make
our world a bit safer for fuzz testing.

Discussion: https://postgr.es/m/18483-30bfff42de238000@postgresql.org