fix(test): clear clang-tidy findings in the new suite code

- JobQueue_test: drop the unused <functional>, and make the two
  read-only GaugeFixture instances const. The other two stay mutable
  because they submit jobs through fixture.queue.
- DatabaseConfig_test: drop the unused SystemParameters.h and include
  ByteUtilities.h for megabytes(), which was reached only transitively.

Regenerate ordering.txt for the resulting edges: test.unit_test arrives
with SuiteJournal.h, and xrpl.protocol leaves with SystemParameters.h.

.github/scripts/levelization/results/ordering.txt

@@ -135,11 +135,11 @@ test.ledger > xrpl.json
 test.ledger > xrpl.ledger
 test.ledger > xrpl.protocol
 test.nodestore > test.jtx
+test.nodestore > test.unit_test
 test.nodestore > xrpl.basics
 test.nodestore > xrpl.config
 test.nodestore > xrpld.core
 test.nodestore > xrpl.nodestore
-test.nodestore > xrpl.protocol
 test.nodestore > xrpl.rdb
 test.overlay > test.jtx
 test.overlay > test.unit_test