refactor: Enable more clang-tidy readability checks (#6595)

Co-authored-by: Sergey Kuznetsov <kuzzz99@gmail.com>

src/test/app/Batch_test.cpp

@@ -35,7 +35,7 @@ class Batch_test : public beast::unit_test::suite
         std::string txHash;
     };
 
-    Json::Value
+    static Json::Value
     getTxByIndex(Json::Value const& jrr, int const index)
     {
         for (auto const& txn : jrr[jss::result][jss::ledger][jss::transactions])
@@ -46,7 +46,7 @@ class Batch_test : public beast::unit_test::suite
         return {};
     }
 
-    Json::Value
+    static Json::Value
     getLastLedger(jtx::Env& env)
     {
         Json::Value params;
@@ -125,7 +125,7 @@ class Batch_test : public beast::unit_test::suite
         return p;
     }
 
-    auto
+    static auto
     openLedgerFee(jtx::Env& env, XRPAmount const& batchFee)
     {
         using namespace jtx;