Add EXPLAIN tests and fix bug

[NGA-TRAN]

You can refer to the query tests in basic_tests.rs from this PR as a template. To get started, simply create a new file called basic_explain_tests.rs and replicate the test structure—just prepend EXPLAIN to the query. A preliminary check suggests that the EXPLAIN output is returning query results instead of execution plans (logical, physical, distributed plans, and stages). This may be a bug in either the code or the test configuration.

[gabotechs]

Shipped in #78 and #68