Refactor the unit test directory structure

Changes

• Moved some common Cargo dependencies used by unit tests to the workspace root.
• Deleted unused features and dependencies in examples and unit tests.
• Moved UniFFI unit tests from tests to tests/uniffi.
• Created crate uniffi-kmm-fixture-build-common in tests/build-common containing the scaffolding generation logic used by all unit tests.
• Moved Rust files from uniffi to other directories in each unit test, using the same structure in examples/.
  • tests/uniffi/<test name>/uniffi/* -> tests/uniffi/<test name>/*
  • tests/uniffi/<test name>/uniffi/src/<test name>.udl -> tests/uniffi/<test name>/src/<test name>.udl
  • tests/uniffi/<test name>/uniffi/src/*.rs -> tests/uniffi/<test name>/src/commonMain/rust/*.rs