This PR utilizes GitHub Action's matrix's to automatically distribute
the CI testlib gem5 build and test jobs across available GitHub Action
Runners.
The CI tests (the `quick` testlib tests, i.e. those run with `./main.py
run`) are distributed across the runners on a per directory basis ---
all directories under "tests/gem5" are run as their own jobs.
The necessary gem5 builds for each workflow are now automatically
inferred via the introduction of `./main.py list`'s `--build-targets`
flag which returns the gem5 build target for a given test or collection
of tests. E.g., `./main.py list --build-targets` will return the build
targets for all the `quick` testlib tests and `./main.py list
--build-target --uid=<id>` will return the build targets the test suite
`<id>` requires.
Moving from monolithic jobs to fine-grained ones will make the locaiton
of test failures more obvious. Each job has it's own artifact containing
"test/testing-results" for the tests run in that job. In addition,
maintenance of these files should become less burdensome due to less
hardcoding.
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