Fix HttpSys Caching_SendFileWithFullContentLength_Cached test (#4630)

Author: Tratcher

src/Servers/HttpSys/test/FunctionalTests/ResponseCachingTests.cs

@@ -21,7 +21,7 @@ public class ResponseCachingTests
 
         public ResponseCachingTests()
         {
-            _absoluteFilePath = Directory.GetFiles(Directory.GetCurrentDirectory()).First();
+            _absoluteFilePath = Path.Combine(Directory.GetCurrentDirectory(), "Microsoft.AspNetCore.Server.HttpSys.dll");
             _fileLength = new FileInfo(_absoluteFilePath).Length;
         }