Fix failing CI tests

[Alexsp32]

MKL_jll v2024.0.0 fails to compile on several systems I've tried. Package dependencies should be set up to prevent installation, but resolving versions occasionally fails and installs this version anyway.

[jamesgardner1421]

If possible I would try to avoid using the separate Project.toml file in the test directory. Although the Julia docs makes it seem like this is the newer recommended method, it breaks all the time. Every other package I've seen on github still uses the old format with the [extras] field so I would suggest still doing that. You should be able to specify versions in the usual way in the [deps] section.
What is the problem with the new MKL? Is this a problem that everyone is having?

[Alexsp32]

I've pinned MKL_jll.jl < 2024 within the main Project file now, which was thankfully possible with all existing dependencies.

I believe what's happening is that the main dependencies resolve MKL_jll@v2024 via LinearSolve.jl.
Requiring MKL.jl for tests should downgrade MKL_jll<v2024 as per compat entries for MKL.jl, but that doesn't appear to happen.