IntegrationTests: actually fail test if tests aren't configured

Looks like calling `XCTFail` followed by throwing an error was making the test continue for some reason, which was then making it crash instead because `Purchases.shared` isn't set.
By removing the `XCTFail`, `XCTest` now correctly sees this as a test failure and doesn't run the tests.

See also RevenueCat/purchases-hybrid-common#210

Tests/BackendIntegrationTests/BaseBackendIntegrationTests.swift

@@ -47,8 +47,7 @@ class BaseBackendIntegrationTests: XCTestCase {
         self.continueAfterFailure = false
 
         guard Constants.apiKey != "REVENUECAT_API_KEY", Constants.proxyURL != "REVENUECAT_PROXY_URL" else {
-            XCTFail("Must set configuration in `Constants.swift`")
-            throw ErrorCode.configurationError
+            throw ErrorUtils.configurationError(message: "Must set configuration in `Constants.swift`")
         }
 
         userDefaults = UserDefaults(suiteName: Constants.userDefaultsSuiteName)