Prevent instability in contrib/pageinspect's regression test.

Prevent instability in contrib/pageinspect's regression test.

pageinspect has occasionally failed on slow buildfarm members,
with symptoms indicating that the expected effects of VACUUM
FREEZE didn't happen.  This is presumably because a background
transaction such as auto-analyze was holding back global xmin.

We can work around that by using a temp table in the test.
Since commit a7212be8b, that will use an up-to-date cutoff xmin
regardless of other processes.  And pageinspect itself shouldn't
really care whether the table is temp.

Back-patch to v14.  There would be no point in older branches
without back-patching a7212be8b, which seems like more trouble
than the problem is worth.

Discussion: https://postgr.es/m/2892135.1668976646@sss.pgh.pa.us