test_speculation_ctrl will fail on some systems

[Spencer Baugh]

test_speculation_ctrl calls prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_ENABLE), but when running on a kernel where someone has previously called prctl(PR_SET_SPECULATION_CTRL, PR_SPEC_STORE_BYPASS, PR_SPEC_FORCE_DISABLE), this will always fail.

We ran into this in the Nix package, where we need to disable this test to build reliably: NixOS/nixpkgs#106875

Maybe the test should call PR_GET_SPECULATION_CTRL first and check if it's PR_SPEC_FORCE_DISABLE, and skip the ENABLE if so?