postgres_fdw: Add more test coverage for EvalPlanQual testing

Hi,

In [1] it was pointed out that there is no test coverage for
postgresRecheckForeignScan. So I would like to propose to add test
cases to cover that function (and related core functions like
ForeignRecheck), as promised in that thread. Please find attached a
patch. (Note that commit 12609fbac, which fixes an EPQ issue reported
in [1], added a test case for ExecScanFetch(), but didn't add any test
cases for that function.)

In the patch I modified all permutations, including existing one, to
use per-session setup doing "BEGIN ISOLATION LEVEL READ COMMITTED",
for simplicity, and modified existing step names/comments a little bit
to match new ones, for consistency.

Like commit 12609fbac, I would also like to propose to back-patch this
to all supported versions.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAD21AoBpo6Gx55FBOW+9s5X=nUw3Xpq64v35fpDEKsTERnc4TQ@mail.gmail.com