sql: postgres diff tool detects indexes as tables

[mnovelodou]

Describe the problem
Diff tool is reporting missing tables tables such as pg_aggregate_fnoid_index as missing tables

To Reproduce

1. Remove the expected diffs (pkg/sql/testdata/pg_catalog_test_expected_diffs.json)
2. Run diff tool, you'll get pg_aggregate_fnoid_index and other indexes reported

Expected behavior
Diff tool should report only tables

Additional data / screenshots

Environment:

• CockroachDB version: master branch
• Server OS: OSX
• Client app: Unit Test

Additional context
What was the impact?
We are adding indexes as tables in the pg_catalog

Add any other context about the problem here.
Need to update the query that retrieves the tables in postgres

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• @solongordon (found keywords: pg_)
• @rafiss (found keywords: pg_)

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