Replace diff test in abi-gen with CI check

[xianny]

Current Behavior

abi-gen currently has a copy of generated code checked in at abi-gen/test-cli/expected-output. The yarn build step for this package will generate fresh code at abi-gen/test-cli/output (gitignored) and fail a test if it differs from the expected-output. Devs have to manually copy output to expected-output when making any changes that ripple through the generated output. This results in annoying CI failures unless devs are familiar with the unusual workflow.

Possible Solution

Get rid of the expected-output folder. Remove output from gitignore. The build step will continue to update output and keep it in sync with any generator changes. Unit tests continue to rely on output.

Add a CI step after building that compares freshly generated code to the output folder at the current commit.
abi-gen$ git diff test-cli/output
If there are diffs, it fails. This enforces that changes to generator and generated are always in sync, even if devs deviate from the yarn build workflow, e.g. by manually generating wrappers.

Devs should manually review changes to output in their PRs.