[llvm] [llvm][test][CGPluginTest] Fix missing `plugins` in test requirements (PR #215010)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Shouldn't plugins also work without llvm-dylib? (At least for plugins using the new API, this is possible.) LGTM otherwise.

https://github.com/llvm/llvm-project/pull/215010