[TODO] Use `optee-utee-mock` in unit testing of LoadablePlugin

[ivila]

Currently LoadablePlugin use a custom C function in unit testing: https://github.com/apache/incubator-teaclave-trustzone-sdk/blob/7dafe79662ee9bf1274aa1ae3efe93cef1782398/optee-utee/src/extension.rs#L239-L246

Should use optee-utee-mock instead.