Also verify stderr matches between wslc and wslc.exe

Address PR feedback: assert that stderr output is identical between
the shell script and direct .exe invocation, fixing unused variable
warnings that would fail MSVC /WX builds.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Ben Hillis

test/windows/SimpleTests.cpp

@@ -302,6 +302,7 @@ class SimpleTests
         auto [exeOutput, exeErr] = LxsstuLaunchWslAndCaptureOutput(L"wslc.exe version");
         auto [scriptOutput, scriptErr] = LxsstuLaunchWslAndCaptureOutput(L"wslc version");
         VERIFY_ARE_EQUAL(exeOutput, scriptOutput);
+        VERIFY_ARE_EQUAL(exeErr, scriptErr);
     }
 };
 } // namespace SimpleTests