Add testOnlyFakeProposal for testing purposes outside SDK

Sources/ZcashLightClientKit/Model/Proposal.swift

@@ -16,3 +16,18 @@ public struct Proposal: Equatable {
         Zatoshi(Int64(inner.balance.feeRequired))
     }
 }
+
+public extension Proposal {
+    /// IMPORTANT: This function is for testing purposes only. It produces fake invalid
+    /// data that can be used to check UI elements, but will always produce an error when
+    /// passed to `Synchronizer.createProposedTransactions`. It should never be called in
+    /// production code.
+    static func testOnlyFakeProposal(totalFee: UInt64) -> Self {
+        var ffiProposal = FfiProposal()
+        var balance = FfiTransactionBalance()
+
+        balance.feeRequired = totalFee
+
+        return Self(inner: ffiProposal)
+    }
+}