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/app/AccountSet_test.cpp

@@ -307,9 +307,13 @@ public:
 
                     // If the field is not present expect the default value
                     if (!(*env.le(alice))[~sfTransferRate])
+                    {
                         BEAST_EXPECT(r.get == 1.0);
+                    }
                     else
+                    {
                         BEAST_EXPECT(*(*env.le(alice))[~sfTransferRate] == r.get * QUALITY_ONE);
+                    }
                 }
             };