Fix #21209 (#21237)

Fix #21209 (#21237)

This commit has two changes to Native Load Library Tests fix the failures in:
https://github.com/dotnet/coreclr/issues/21209

1) Windows: The test tried to load a DLL from Win32 directory which doesn't exist
   in Windows server 2016. So changed to use a file that exists.
2) Linux: Disable the test when run against packages -- because in this configuration
   the tests are built on Windows, and the native DLL is built separately and copied
   over to the CORE_ROOT directory instead of the test directory.
   [Several other tests are disabled with the comment "Issue building native DLL" on Linux]