Investigate pg_regress diffs

We recently started running the pg_regress test suite as a roachtest, but still need to closely examine all the diffs from the tests in the suite to check that all the diffs between postgres and CRDB are expected and not a compatibility bug.

The following spreadsheet tracks the tests that have been investigated so far: https://docs.google.com/spreadsheets/d/1f52itpzKOHRCMG0zkiawHke_ZqEYZYbnKHYtlzzf3ws/edit#gid=0

The test diffs can be found in pkg/cmd/roachtest/testdata/regression.diffs. Postgres expected output from the tests are [located here](https://github.com/postgres/postgres/tree/d053a879bb360fb72c46de2a013e741d3f7c9e8d/src/test/regress/expected).

Jira issue: CRDB-35760