Update ValidVault to allow LoanPay to change LossUnrealized

- Making a payment on an impaired loan will unimpair the loan, which
  changes Vault.LossUnrealized.
- Add a step in unit tests to impair a loan before making a payment, to
  verify, and prevent future similar regressions.
- Resolves regression RIPD-3650

src/test/app/Loan_test.cpp

@@ -1856,6 +1856,12 @@ class Loan_test : public beast::unit_test::suite
                     auto const borrowerBalanceBeforePayment =
                         env.balance(borrower, broker.asset);
 
+                    if (canImpairLoan(env, broker, state))
+                        // Making a payment will unimpair the loan
+                        env(manage(lender, loanKeylet.key, tfLoanImpair));
+
+                    env.close();
+
                     // Make the payment
                     env(pay(borrower, loanKeylet.key, transactionAmount));
 

src/xrpld/app/tx/detail/InvariantCheck.cpp

@@ -2857,7 +2857,7 @@ ValidVault::finalize(
 
     if (!beforeVault_.empty() &&
         afterVault.lossUnrealized != beforeVault_[0].lossUnrealized &&
-        tx.getTxnType() != ttLOAN_MANAGE)
+        txnType != ttLOAN_MANAGE && txnType != ttLOAN_PAY)
     {
         JLOG(j.fatal()) <<  //
             "Invariant failed: vault transaction must not change loss "