Make test_display_pg_json pass regardless of build setup and dependencies

[AdamGS]

Which issue does this PR close?

• Closes #test_display_pg_json currently fails with default features #21501.

Rationale for this change

Running tests for datafusion-expr currently fails locally, but due to feature unification with the subtrait dependency it does pass in CI.

What changes are included in this PR?

Enables a feature for a dependency during tests.

Are these changes tested?

Ran tests for the crate directly.

Are there any user-facing changes?

None.

[AdamGS]

Just realized that there's a SLT test that has the same question in explain.slt:642, it just runs explain format pgjson select * from values (1);. I'll add the same fix to the sqllogictest crate, I think its best to avoid enabling it globally because its a pretty popular crate and we don't want to impact downstream users.