document and narrow usage of criu tests

I'm unable to run `https://github.com/adoptium/aqa-tests/tree/master/external/*criu*` tests. Is there some additional documentation? If no, can it be written?

In addition, it seems that those test are bound to only very specific subset of jvms, and images. That is causing quite intensive `if-elif-elif-elif....fi.fi.fi` in the external scripts.
* Instead of this, can they simply exit if incorrect combination is set?
* There seems to be mechanism, which allows to move to much vendor-tight tests out of main repo. Is this applicable here?
* The main runners would lost osme weight

Thoughts please? Maye I'm just missing some part of history or documentation, but some pointers would be helpfull

Start by reading the tests under external/*criu* and the related external scripts to identify their prerequisites and supported JVM/image combinations. Document how to run them, clarify which combinations are valid, and describe whether unsupported combinations should exit or move to vendor-specific tests. Done means the usage and scope are explicit for the main runners.

Written by the indexing model from the issue text.