Backport to branch(3.16) : Minor fixes to unit tests (#2828)

Co-authored-by: Toshihiro Suzuki <[email protected]>

Author: feeblefakie

core/src/test/java/com/scalar/db/transaction/consensuscommit/CoordinatorGroupCommitterTest.java

@@ -258,8 +258,9 @@ void remove_GivenAllFullTxIds_ShouldRemoveAll() throws Exception {
     }
   }
 
+  @SuppressWarnings("ClassCanBeStatic")
   @Nested
-  static class CoordinatorGroupCommitKeyManipulatorTest {
+  class CoordinatorGroupCommitKeyManipulatorTest {
     private final CoordinatorGroupCommitKeyManipulator keyManipulator =
         new CoordinatorGroupCommitKeyManipulator();