Instead of overriding the process stderr to get
the string representation from mlir we can can
directly capture in into a string using mlir's
printOperation.
Another problem with overriding stderr is that
each `#[test]` runs as a different thread meaning that
as soon as we have 2+ tests the tests could panic
due to conflicts/races between the different overrides.
This also moves the expected string directly into the test
as a literal.
The rust bindings are intented to access both LLVM/MLIR CAPI as well as
the concrete-compiler one. This initial commit provide the API for
LLVM/MLIR only. Tests should be used as an example to how to generate a
valid DAG of operations in MLIR.
