fix test for linux

Author: Stuart Espey

Xcode/Tests/ServerRawResponseTests.swift

@@ -62,12 +62,13 @@ class ServerRawResponseTests: XCTestCase {
                         let statusCode = response?.statusCode
                         XCTAssertNotNil(statusCode)
                         XCTAssertEqual(statusCode, 200)
+                    #if !os(Linux)
                         if #available(iOS 13.0, macOS 10.15, tvOS 13.0, *) {
                             let header = response?.value(forHTTPHeaderField: "XXX-Custom-Header") ?? ""
                             XCTAssertEqual(header, testString)
-                        } else {
-                            // Fallback on earlier versions
                         }
+                    #endif
+
                         XCTAssertEqual(body, testString.data(using: .utf8))
                     }