C#: Make path relative to testdir explicit in CWE-601 testcase.

Author: michaelnebel

csharp/ql/test/query-tests/Security Features/CWE-601/UrlRedirect/options

@@ -1,4 +1,4 @@
 semmle-extractor-options: /nostdlib /noconfig
 semmle-extractor-options: --load-sources-from-project:${testdir}/../../../../resources/stubs/_frameworks/Microsoft.NETCore.App/Microsoft.NETCore.App.csproj
-semmle-extractor-options: --load-sources-from-project:../../../../resources/stubs/_frameworks/Microsoft.AspNetCore.App/Microsoft.AspNetCore.App.csproj
+semmle-extractor-options: --load-sources-from-project:${testdir}/../../../../resources/stubs/_frameworks/Microsoft.AspNetCore.App/Microsoft.AspNetCore.App.csproj
 semmle-extractor-options: ${testdir}/../../../../resources/stubs/System.Web.cs