refactor: Add simple clang-tidy readability checks (#6556)

This change enables the following clang-tidy checks:
-  readability-avoid-nested-conditional-operator,
-  readability-avoid-return-with-void-value,
-  readability-braces-around-statements,
-  readability-const-return-type,
-  readability-container-contains,
-  readability-container-size-empty,
-  readability-else-after-return,
-  readability-make-member-function-const,
-  readability-redundant-casting,
-  readability-redundant-inline-specifier,
-  readability-redundant-member-init,
-  readability-redundant-string-init,
-  readability-reference-to-constructed-temporary,
-  readability-static-definition

src/test/rpc/ValidatorRPC_test.cpp

@@ -134,7 +134,7 @@ public:
                     parseBase58<PublicKey>(TokenType::NodePublic, jrrnUnl[x].asString());
                 BEAST_EXPECT(parsedKey);
                 if (parsedKey)
-                    BEAST_EXPECT(disabledKeys.find(*parsedKey) != disabledKeys.end());
+                    BEAST_EXPECT(disabledKeys.contains(*parsedKey));
             }
 
             disabledKeys.clear();