pgsql: Add more test coverage for pg_clear_attribute_stats()

Add more test coverage for pg_clear_attribute_stats()

pg_clear_attribute_stats() was called exactly once in the whole test
suite, only for a correct cleanup. None of its failure scenarios were
covered, in terms of:
- NULL input values
- Incorrect object names (schema, attribute, relation)
- Incorrect relkinds
- Inherited pg_statistic entry cleanup

All these cases are now covered. This would have caught the failure
fixed by 11ed011ae22, as one example.

Author: Zhong ShiHao <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAGRkXqQEFzVh_bQSoLGDqaRTq7Xta6OUoda8PDrWDPWcAuwk=w@mail.gmail.com

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/66f85615d5d650c47b6e8ea75e32a6817cb23dac

Modified Files
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src/test/regress/expected/stats_import.out | 137 +++++++++++++++++++++++++++++
src/test/regress/sql/stats_import.sql | 96 ++++++++++++++++++++
2 files changed, 233 insertions(+)