Writing noirc_frontend Unit Tests

Tests live under compiler/noirc_frontend/src/tests/. The root module tests.rs defines helpers and declares submodules organized by topic.

Helpers in `tests.rs`

Importable from submodules via use crate::tests::{...}.

Helper	Use when
`assert_no_errors(src)`	Program should compile without errors. Panics with `"Expected no errors"` and prints diagnostics. Returns `Context`.
`assert_no_errors_without_report(src)`	Same but doesn't print diagnostics on failure.
`get_program_errors(src) -> Vec<CompilationError>`	Get raw error list for manual inspection.
`get_program_using_features(src, &[UnstableFeature])`	Compile with specific unstable features enabled.