[SPARK-51743] Add describe_(database|table), show_(database|table), explain sql test and answer files

[dongjoon-hyun]

What changes were proposed in this pull request?

This PR aims to add the following test coverage.

DESCRIBE DATABASE
DESCRIBE TABLE
EXPLAIN
SHOW DATABASE
SHOW TABLE

Why are the changes needed?

To improve the test coverage.

Does this PR introduce any user-facing change?

No, this is a test-only addition.

How was this patch tested?

Pass the CIs and do manual check.

$ swift test --filter SQLTests
...
􀟈  Suite SQLTests started.
􀟈  Test runAll() started.
􀟈  Test testRemoveID() started.
􁁛  Test testRemoveID() passed after 0.006 seconds.
binary.sql
cache.sql
clear_cache.sql
date.sql
describe_database.sql
describe_function.sql
describe_query.sql
describe_table.sql
explain.sql
floating.sql
integral.sql
pipesyntax.sql
select.sql
show_databases.sql
show_tables.sql
string.sql
uncache.sql
􁁛  Test runAll() passed after 0.529 seconds.
􁁛  Suite SQLTests passed after 0.529 seconds.
􁁛  Test run with 2 tests passed after 0.529 seconds.

Was this patch authored or co-authored using generative AI tooling?

No.

[dongjoon-hyun]

Thank you, @peter-toth ! Merged to main.