Split up `gml/tests/interpreter.rs`

[rpjohnst]

Currently, all the GML tests are in a single file, implemented as #[test] functions. This makes them annoying to read, and leads to large and repetitive diffs when the API for running GML changes.

Compiler test suites often work with a single test per file, with test metadata in special comments. This makes tests easier to read and write, and consolidates the implementation of "how to run a test" in one place. Examples include Rust's compiletest or LLVM's FileCheck.