Exemplo de uma forma mais correta no uso de protocolos e testes

CurrencyConverterCorrect.swift

import Foundation

protocol ExchangeProtocol {
    var value: Double { get }
}

protocol Taxable {
    var value: Double { get }
    func calculated(exch: ExchangeProtocol) -> Double
}

extension Taxable {
    func calculated(exch: ExchangeProtocol) -> Double {
        return exch.value*(value/100)
    }
}

class CurrencyConverter {
    
    private var exchange: ExchangeProtocol
    private var tax: Taxable
    
    init(exchange: ExchangeProtocol, tax: Taxable) {
        self.exchange = exchange
        self.tax = tax
    }
    
    func converter(value: Double) -> Double {
        return (value*exchange.value)-tax.calculated(exch: exchange)
    }
}

// Testes

import XCTest

struct DollarMock: ExchangeProtocol {
    var value: Double
}

struct TaxMock: Taxable {
    var value: Double
    
    func calculated(exch: ExchangeProtocol) -> Double {
        return 0
    }
}

class CurrencyConverterTest: XCTestCase {
    
    func testConverter_ShouldSuccess() {
        let dollarMock = DollarMock(value: 6)
        let taxMock = TaxMock(value: 1.5)
        
        let currencyConverter = CurrencyConverter(exchange: dollarMock, tax: taxMock)
        let amountConverter = currencyConverter.converter(value: 1)
        XCTAssertEqual(amountConverter, 6)
    }
}

CurrencyConverterTest.defaultTestSuite.run()