[WIP] Migrate matmul test suite to iree-test-suites.

[ScottTodd]

** Work in progress **

Progress on https://github.com/iree-org/iree-test-suites/issues/2.

I'm moving the tests at tests/e2e/matmul to the iree-org/iree-test-suites repository in https://github.com/iree-org/iree-test-suites/pull/13. This deletes them from this repo and adds back test coverage using some workflow changes.

TODO:

• [ ] Port conv and attention tests too (this deletes them without adding them back yet)
• [ ] Add CPU test job
• [ ] Add tests to other GPU test jobs
• [ ] Iterate on the CMake options, flags, helper scripts, etc. so developers can still use these tests with minimal changes to their local workflows (this sort of move exposes a lot of the accumulated cruft and some deeper changes are overdue)
• [ ] Add docs to https://iree.dev/developers/general/testing-guide/

ci-exactly: build_packages, test_amd_mi250