Running tests on `cdylib` driver crates cause failure in Github Actions Runner

[wmmc88]

Example failure:

     Running `D:\a\windows-drivers-rs\windows-drivers-rs\tests\mixed-package-kmdf-workspace\target\debug\deps\driver-3c34d004f375bef0.exe`
error: test failed, to rerun pass `-p driver --lib`

Caused by:
  process didn't exit successfully: `D:\a\windows-drivers-rs\windows-drivers-rs\tests\mixed-package-kmdf-workspace\target\debug\deps\driver-3c34d004f375bef0.exe` (exit code: 0xc0000005, STATUS_ACCESS_VIOLATION)

This only happens with KM drivers. The test build ends up linking against KM libraries that cause the above failure (since tests run in UM).

This is odd since it only seems to manifest in github actions runners. Even on a local machine that is running a windows-server 2025 image does not repro this issue.

[wmmc88]

One possible approach to this is to have the empty extern blocks with the link attribute be generated in wdk-sys and tie it to cfg(not(feature=test-stubs)). This should essentially be the same thing as what we currently do (ie. emit cargo::rustc-link-lib=static prints), but now we can prevent linking when compiling for tests