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

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

src/test/protocol/PublicKey_test.cpp

@@ -46,7 +46,7 @@ public:
         }
     }
 
-    blob
+    static blob
     sig(std::string const& hex)
     {
         blob b;
@@ -54,7 +54,7 @@ public:
         return b;
     }
 
-    bool
+    static bool
     check(std::optional<ECDSACanonicality> answer, std::string const& s)
     {
         return ecdsaCanonicality(makeSlice(sig(s))) == answer;

src/test/protocol/STAmount_test.cpp

@@ -21,7 +21,7 @@ public:
     }
 
     //--------------------------------------------------------------------------
-    STAmount
+    static STAmount
     roundSelf(STAmount const& amount)
     {
         if (amount.native())

src/test/protocol/STParsedJSON_test.cpp

@@ -11,7 +11,7 @@ namespace xrpl {
 
 class STParsedJSON_test : public beast::unit_test::suite
 {
-    bool
+    static bool
     parseJSONString(std::string const& json, Json::Value& to)
     {
         Json::Reader reader;

src/test/protocol/SeqProxy_test.cpp

@@ -42,7 +42,7 @@ struct SeqProxy_test : public beast::unit_test::suite
     }
 
     // Verify streaming.
-    bool
+    static bool
     streamTest(SeqProxy seqProx)
     {
         std::string const type{seqProx.isSeq() ? "sequence" : "ticket"};