Provide plpgsql tests for cases involving record field changes.

Provide plpgsql tests for cases involving record field changes.

We suppressed one of these test cases in commit feb1cc559 because
it was failing to produce the expected results on CLOBBER_CACHE_ALWAYS
buildfarm members.  But now we need another test with similar behavior,
so let's set up a test file that is expected to vary between regular and
CLOBBER_CACHE_ALWAYS cases, and provide variant expected files.

Someday we should fix plpgsql's failure for change-of-field-type, and
then the discrepancy will go away and we can fold these tests back
into plpgsql_record.sql.  But today is not that day.

Discussion: https://postgr.es/m/87wotkfju1.fsf@news-spur.riddles.org.uk