Adding Valgrind workflow for GitHub Actions

[sjoblomj]

This PR adds a GitHub Actions Workflow that runs Valgrind (memory leak checker) on each PR.

The PR also adds pretty useless tests for the about and version subcommands. With those tests we have a complete test suite for all subcommands. That also means Valgrind gets to check all paths.

I created this PR using AI - it all seems sensible to me, but I am not an expert at either Valgrind nor GitHub Actions Workflows.

Running locally

# Build image
docker build --build-arg USER_ID=$(id -u) --build-arg GROUP_ID=$(id -g) -t mpqcli-valgrind -f Dockerfile.valgrind .

# Run image against the list subcommand and the Patch_rt.mpq file
docker run --rm \
  -v $(pwd)/path/to/directory/with/test/mpqs:/mpqcli/test/data:ro \
  mpqcli-valgrind \
  valgrind --leak-check=full --show-leak-kinds=all \
  ./build/bin/mpqcli list /mpqcli/test/data/Patch_rt.mpq

Maybe this should be mentioned in the Readme. Up to you :)

[sjoblomj]

The job fails because there are memory leaks. #149 fixes those, so after that is merged, this job should turn green.

[sjoblomj]

One of the steps in the GitHub Workflow will post comments on the PR if it contains memory leaks. But it currently does not have permissions to do so. You'll need to grant those permissions to the repo. AI says:

1. Go to your repository on GitHub: https://github.com/TheGrayDot/mpqcli
2. Click Settings (top right)
3. In the left sidebar, click Actions → General
4. Scroll down to Workflow permissions
5. Select "Read and write permissions" (instead of "Read repository contents and packages permissions")
6. Check the box "Allow GitHub Actions to create and approve pull requests"
7. Click Save

[sjoblomj]

Not sure if this is wanted or needed?