refactor: Restructure Transactor::preflight to reduce boilerplate (#5592)

* Restructures `Transactor::preflight` to create several functions that will remove the need for error-prone boilerplate code in derived classes' implementations of `preflight`.

include/xrpl/protocol/TER.h

@@ -673,7 +673,8 @@ isTerRetry(TER x) noexcept
 inline bool
 isTesSuccess(TER x) noexcept
 {
-    return (x == tesSUCCESS);
+    // Makes use of TERSubset::operator bool()
+    return !(x);
 }
 
 inline bool