move mir-opt miri tests to CI logic

[RalfJung]

It is useful to run Miri tests with mir-opts as that sometimes finds miscompilations.

However, doing so on every ./x test miri is annoying as it makes that take a lot longer, and it is inconsistent with ./miri test in the Miri repo. So remove this logic there, and instead have the check-miri script in CI invoke the test suite a 2nd time with mir-opts enabled.

Cc @rust-lang/miri

[rustbot]

r? @Mark-Simulacrum

rustbot has assigned @Mark-Simulacrum.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: bootstrap
• bootstrap expanded to 6 candidates
• Random selection from Mark-Simulacrum, clubby789, jieyouxu