fix Swift Package Index build (swift-server#369)

Co-authored-by: Johannes Weiss <[email protected]>

Author: weissi

Tests/AsyncHTTPClientTests/HTTPClientNIOTSTests.swift

@@ -115,14 +115,19 @@ class HTTPClientNIOTSTests: XCTestCase {
     func testTrustRootCertificateLoadFail() {
         guard isTestingNIOTS() else { return }
         #if canImport(Network)
-            let tlsConfig = TLSConfiguration.forClient(trustRoots: .file("not/a/certificate"))
-            XCTAssertThrowsError(try tlsConfig.getNWProtocolTLSOptions()) { error in
-                switch error {
-                case let error as NIOSSL.NIOSSLError where error == .failedToLoadCertificate:
-                    break
-                default:
-                    XCTFail("\(error)")
+            if #available(macOS 10.14, iOS 12.0, tvOS 12.0, watchOS 6.0, *) {
+                let tlsConfig = TLSConfiguration.forClient(trustRoots: .file("not/a/certificate"))
+
+                XCTAssertThrowsError(try tlsConfig.getNWProtocolTLSOptions()) { error in
+                    switch error {
+                    case let error as NIOSSL.NIOSSLError where error == .failedToLoadCertificate:
+                        break
+                    default:
+                        XCTFail("\(error)")
+                    }
                 }
+            } else {
+                XCTFail("should be impossible")
             }
         #endif
     }