Filtering tests does not work any more

[RalfJung]

./miri test foo used to only run the tests that have "foo" in their name (including the folder name, i.e. "fail" matches all compile-fail/foo.rs tests). That doesn't work any more.
I use this quite a lot when testing so would be nice to get it back. :)

Cc @oli-obk

[oli-obk]

Oh yea, I knew this was lost but forgot to add it to the list

[RalfJung]

Once this is fixed we should also enable running tests with optimized MIR again, filtered for "tests/pass" and "tests/panic".