Add -t / --test override for execute

[comps]

Currently, we are able to split tmt execution by simply specifying the steps one-at-a-time, using --before, --after, or just manually specifying them, ie.

$ tmt \
    -c distro=rhel-9.5 -c arch=x86_64 \
    run -i /var/tmp/tmt/unique_test_dir \
        plans -n /plans/default \
        discover -h fmf \
        prepare -e SOME_VAR=123 \
        provision -h connect -k ~/.ssh/id_rsa -u user -g some_hostname

$ tmt \
    run -i /var/tmp/tmt/unique_test_dir \
        execute -h tmt

...

and this would be great in a use case where we want to execute multiple tests, possibly one-at-a-time, on the given guest, ie. when using snapshotting or re-running on failure, or just having multiple (dynamically added/removed) guests and load-balancing test execution amongst them.

This is doable by specifying all tests for discover (so they are all present in discover/tests.yaml), but then there is no way to run only one of those tests for execute, we would need to restart discover + prepare (which can take a long time).

Would it be possible to add some "filter" to execute -h tmt that would simply further filter discover/tests.yaml with the given expression? Kind of like discover is able to do --test / --filter / --exclude on top of what the plan specifies?

(Yes, the obvious hack is to modify tests.yaml, but I was hoping for a more standard solution.)

Thanks!