modify rez-selftest -s arg

Update the rez-selftest -s arg to take multiple shells, and for each of those, assert that the shell is available, rather than skip over it if not.

Right now it isn't that clear whether a given shell is actually getting tested in the CI; for eg, rez could think the shell is unavailable (which in itself may be a bug). So the way I'd see it working is, we would do the apt-get install etc of the shells we want to test, then pass those shells to rez-selftest also, which would fail if any of those shells are not available / rez doesn't know about them.

Start at the rez-selftest -s argument handling and review how the CI installs and passes shells to it. Update the behavior to accept multiple shells and fail when any requested shell is unavailable or unrecognized; done means each installed CI shell is explicitly asserted rather than skipped.

Written by the indexing model from the issue text.