Move core tests from `codegen-client` into `codegen-core` (#1776)

* Move `testutil` into `core` where possible
* Move `rustlang` tests into `core`
* Move remaining `util` tests to `core`
* Move and refactor `TopLevelErrorGeneratorTest` to not require `RustCodegenPlugin`
* Move protocols into `core`
* Move `generator` tests into `core`
* Move protocol ser/de tests into `core`
* Suppress deprecation warning in `TopLevelErrorGeneratorTest`
* Move `InlineFunctionNamerTest` into `core`
* Move model transform tests into `core`
* Rename `SymbolBuilderTest` to `SymbolVisitorTest` to match class under test
* Move server classes out of client
* Move core codegen tests into their own CI step