Properly set D3D12SDKPath

Closes #5992

This commit have an effect on slang-test.exe and test-server.exe.

When we run slang-test.exe, it uses D3D12Core.DLL located at
`build/{Release,Debug}/bin/D3D12`, which is `D3D12` from where the
executable file is located at.

This has been working fine but we found a case where it doesn't work
properly.  This commit sets D3D12SDKPath value explicitly to avoid
potential problems.

tools/slang-test/slang-test-main.cpp

@@ -47,6 +47,12 @@
 
 using namespace Slang;
 
+#if defined(_WIN32)
+extern "C" { __declspec(dllexport) extern const uint32_t D3D12SDKVersion = 711; }
+
+extern "C" { __declspec(dllexport) extern const char* D3D12SDKPath = u8".\\D3D12\\"; }
+#endif
+
 // Options for a particular test
 struct TestOptions
 {