opt: run sqlsmith roachtest queries with EXPLAIN

This commit modifies the SqlSmith roachtest to run each generated query
with explain. This should allow the test to catch planning errors that
do not happen during normal execution.

Informs cockroachdb#88037

Release note: None

pkg/cmd/roachtest/tests/sqlsmith.go

@@ -228,6 +228,11 @@ INSERT INTO seed_mr_table DEFAULT VALUES;`, regionList[0]),
 					_, err := conn.Exec(stmt)
 					if err == nil {
 						logStmt(stmt)
+						stmt = "EXPLAIN " + stmt
+						_, err = conn.Exec(stmt)
+						if err == nil {
+							logStmt(stmt)
+						}
 					}
 					done <- err
 				}(ctx)